In August at the age of 11, Maguire came joint third in the under-12 World Golf Championship at Pinehurst in North Carolina. Her twin sister Lisa Maguire beat off more than 800 competitors from 30 countries to become the new under-12 world champion. In September, the twins were chosen to bring the Ryder Cup trophy to the stage during the presentation ceremony after Europe's win at the K Club against the USA.
On 6 May, Maguire became the youngest winner of the Hermitage Ladies Scratch Cup at the age of 12. She also won the St. Leonard's Scottish U-16 Open; Connacht Girls U-18 Championship; Ulster U-19 Schools Championship; The Darren Clarke 2007 Girls Championship; Girls Interprovincial Championship and was runner-up in the Midland and Leinster Championships.
In March at the age of thirteen, Maguire won the French Under-18 International title in Paris. On 4 May she successfully defended the Hermitage Ladies Scratch Cup. On 21 May, she won the Lancome Irish Ladies Close Championship at Westport, County Mayo. She beat her twin sister Lisa by 3 and 2 in the final. On 19 July, the positions were reversed when Lisa triumphed over her sister 4 and 3 in the final of the Lancome Irish Girls Close Championship at Mullingar Golf Club. It was the first time in the history of Irish Golf that the Irish Champions of both the Ladies and Girls events came from the same family let alone twin sisters. On 26 July, Leona finished runner-up to her sister who beat her by four shots in the European Young Masters at Chantilly, France. As a result, both twins were picked for the European Junior Ryder Cup Team which later lost to the USA at Olde Stone, Bowling Green, Kentucky on 17 September. In December, Leona and her sister Lisa were named jointly as the Women's Amateur of the Year at the AIB Irish Golf Writers' awards for 2008.
2009 On 13 April, Maguire won the French International Lady Juniors U21 Amateur. On 26 April, at the age of 14, she won the Helen Holm Scottish Women's Open Amateur Strokeplay Golf Championship played at Troon Portland and Troon Old Course after a final round score of 73 for a 54 hole aggregate score of −6, 219. Her sister Lisa came third. On 11 July, the twins were part of the four girl Ireland team which defeated Sweden 4–1 in the final of the Girls' European Team Championships at Kokkola, Finland to become European Champions for the first time in Irish golfing history. She played for Great Britain and Ireland in the Vagliano Trophy in July, becoming the youngest player at the age of 14 ever to represent the side, along with her twin sister, Lisa. The twins were the joint top highest scorers for the GB&I side which lost to Europe 13–11. Both twins were selected for the 2009 Junior Solheim Cup Team at the Aurora Country Club, Illinois, US in August. She won the Irish Ladies Golf Union Order of Merit award for 2009.
On 5 April, Maguire won a second consecutive title at the French Under 21 Open and retained the Esmond Trophy. She played for Great Britain and Ireland in the Curtis Cup in 2010, becoming the youngest player at the age of 15 ever to represent the side, along with her twin sister Lisa. In the same month, she again made more history as the youngest match play qualifier in the British Ladies Amateur over the Royal St David's Links at Harlech, Wales. In September Maguire was awarded the European Ladies' Amateur Golf Ranking's silver medal.
In January, Maguire won the Portuguese Ladies Amateur Open by 15 strokes. In April, she won a second consecutive French Under 21 title. In June, Maguire won the Irish Women's Open Strokeplay title. She played for Great Britain and Ireland in the Vagliano Trophy in June where she won all three of her matches. In August, she won the British Ladies' Open Stroke Play Championship by six shots, the youngest player to ever win the event. She was chosen along with her sister Lisa in the European side which earned a 12–12 draw against the USA in the PING Junior Solheim Cup at Knightsbrook in Meath on 21 September. She scored the maximum three points in the event. Also in September Maguire was awarded the European Ladies' Amateur Golf Ranking's silver medal. In December, Leona and her sister Lisa were named jointly as the Women's Amateur of the Year at the AIB Irish Golf Writers' awards for 2011.
On 22 April, Maguire became the first Irish golfer to win the Irish Girls Open Strokeplay championship, held at Roganstown Golf Club. She beat an international field by eight shots. On 10 June, she was part of the Great Britain & Ireland Curtis Cup team which defeated the USA at Nairn, Scotland by 10.5 points to 9.5 points. On 20 June, Maguire won the Irish Women's Close Amateur Championship at the County Louth Golf Club, for the second time in her career.
On 5 May, Maguire won the Hermitage Scratch Cup at Hermitage Golf Club in Lucan, Dublin.
Maguire left secondary school at Loreto College Cavan in Ireland with top scholastic honours and joined Duke University in Durham, North Carolina, U.S.
Maguire commenced the year in spectacular form in NCAA college golf, shooting up to first place in the World Amateur Golf Ranking by mid-May. In the Darius Rucker Intercollegiate held at Hilton Head, South Carolina, on 8 March, she came first with rounds of 73, 65, 69. In the ACC Championship held at Greensboro, North Carolina on 19 April, she won with rounds of 68, 68, 70. In the NCAA South Regional held at the Warren Golf course, South Bend, Indiana on 9 May, she won again with rounds of 72, 68, 72, thus helping Duke win the NCAA South Bend Regional title. On 6 May, she was voted the ACC (Atlantic Coast Conference) Player of the Year and ACC Freshman of the Year. She was also picked for the ACC All-Star team. She also won ACC Golfer of the Month awards for both March and April. On 25 May, she won the 2015 Annika Award for the best golfer of the year in American college women's golf. On 20 May, she was chosen as a member of the South Bend Region All-Star team. On 26 May Leona was honoured both as the WGCA Player of the Year and Freshman of the Year for 2015 and was also chosen on the 2015 WGCA All-American team. She was also the recipient of the 2014–15 Golfstat Cup presented to the player who has the best scoring average. Maguire's stroke average of 70.78 was the best in the history of Duke University ladies golf. She also tied the Duke record of 24 par rounds or better in a season and tied the Duke record for most wins in a season with three. At the British Ladies Amateur held at Portstewart, County Londonderry, Northern Ireland on 11–13 June, Maguire won the stroke play part of the competition, finishing 6 strokes ahead of the field with rounds of 67, 68. She also helped Ireland to win the international team event. On 25 June Maguire was named on the 2015 All-ACC Academic Team which recognises academic excellence as well as athletic excellence. In June Maguire won the Golfweek National Player of the Year award and also was selected to Golfweek First Team All-America. On 6 July Maguire was named on the WGCA 2015 All-American Scholar Team. On 27 July Maguire represented Great Britain & Ireland in the Vagliano Trophy against Europe, held at Malone Golf Club, Belfast, Northern Ireland. Europe won 12 1/2 points to 11 1/2. Maguire was undefeated, contributing 3 1/2 points of the GB & Ireland total. Maguire was invited as a guest amateur to play in the European Ladies Tour's ISPS Handa Ladies European Masters held at Buckinghamshire Golf Club, Denham, England on 2–5 July. Maguire stunned the professionals by finishing second, just one shot off the lead with rounds of 69, 70, 69, 69. As an amateur Maguire could not accept the 2nd place prize money of €50,000. In her last seven stroke play competitions, she finished 1st, 3rd, 1st, 1st, 2nd, 1st and 2nd. On 20 August 2015, Maguire was awarded the Mark H. McCormack Medal given annually to the number one ranked female amateur player in the world. Maguire was invited to play in the fifth and final major of the year, The Evian Championship held at Evians Golf Club, Evian Les Bains, France, on 10–13 September. Maguire finished as leading amateur and became the first woman ever from the Republic of Ireland to make the cut in a major. As an amateur Maguire could not accept her prize money of €18,648, meaning she has forfeited over €68,000 in prize money already this year. Although still an amateur and having only played a couple of professional tournaments in 2015, Maguire is ranked in the top 300 of the Women's World Golf Rankings. In September Maguire was awarded the European Ladies' Amateur Golf Ranking's gold medal.
Maguire won her fourth tournament of 2015, Ruth's Chris Tar Heel Invite held at Chapel Hill, North Carolina on 9–11 October with rounds of 69, 72, 71. She has now spent six consecutive months on top of the World Amateur Golf Ranking. She was selected as ACC Golfer of the Month for October 2015. On 16 December Maguire was named as Irish Independent Young Sportstar of the Year for 2015. On 20 December Maguire was named as the Irish Golf Writers' Women's Amateur of the Year for 2015.
On 15 March 2016, Maguire was named the ACC Women's Co-Golfer of the Month for February. On 14 April 2016 Maguire completed her 11th consecutive month on top of the world rankings. On 17 May 2016, Maguire was selected for the WGCA All-Region squads for the second year in a row. On 25 May 2016, Maguire was selected for the WGCA All-America first team for the second year in a row. On 12 June 2016 at Dún Laoghaire Golf Club, Maguire was part of the Great Britain & Ireland team which defeated the USA in an 11½ to 8½ win in the 2016 Curtis Cup. She contributed 4 points to the winning total. On 21 June Maguire was selected on the All-ACC Academic Team for the second year in a row. In June Maguire was selected to Golfweek First Team All-America for the second straight season. On 5 July Maguire was selected for the 2015-16 WGCA All-American Scholars Team which recognises academic excellence as well as athletic excellence, for the second year in a row. On 31 July at the Woburn Golf and Country Club in England, Maguire won the Smyth Salver when she finished as the leading amateur at the Ricoh Women's British Open, one of the major championships. As an amateur, she forfeited her 25th place prize money of $25,500, after rounds of 71, 70, 68, 75. In the last 12 months, she has forfeited over $100,000 in prize money as a result of her amateur status, despite playing only a handful of professional tournaments. As a result of the points earned at the British Open, she regained her world number one amateur ranking on 3 August 2016. On 15 August 2016, Maguire was awarded the Mark H. McCormack Medal given annually to the number one ranked female amateur player in the world. This is the second successive season Maguire has won the award. On 20 August Maguire finished 21st overall and was the leading amateur at the Rio Olympics Women's Golf for a two under par total of 282, with scores of 74, 65, 74, 69. On 17 September Maguire helped Ireland to a third place bronze medal in the 27th Women's World Amateur Team Championship Espirito Santo Trophy held at Mayakoka El Camaleon Golf Club in Mexico. Maguire finished in a tie for 6th place in the individual rankings with rounds of 72, 70, 71, 69 for a 6-under-par total of 282. On 28 September Maguire was awarded the European Ladies' Amateur Golf Ranking's gold medal for the second successive year. On 2 November she helped Duke win the East Lake Cup held at East Lake Golf Club in Atlanta, Georgia, USA, beating Washington State in the final.
On 3 August, Maguire announced she would turn professional after the Brazil Olympics and would enter the LPGA Tour qualifying school in October. After successfully completing stage II at qualifying school, Maguire withdrew prior to stage III and stated she would return to Duke.
Maguire started the year with a victory on 14 February in the Northrop Grumman Regional Challenge played at the 6,017-yard, par 71 Palos Verdes Golf Club. She finished joint first with rounds of 68, 71, 70 for her fifth college victory, also helping Duke to third place in the tournament. Maguire added her second victory of the year at the LSU Tiger Golf Classic held at University Club in Baton Rouge, Louisiana, USA on 26 March. She finished with a total of 206 after rounds of 67, 68, 71, which included an eagle. She also helped Duke University win the team title. In her seven tournaments of the 2016–17 college season, Maguire has only finished outside the top three places on one occasion. Her victory equalled the Duke University record of six individual wins held by Brittany Lang and Lindy Duncan. Maguire was selected as ACC Golfer of the Month for March 2017, which is the fifth time she has won the monthly award in her college golf career. Maguire won again on 15 April, claiming the ACC Championship for the second time in her career. She finished with a total of 210 after rounds of 68, 72 & 70 at the Reserve Golf Club in Pawleys Island, South Carolina, USA. She also helped Duke University to win the ACC team title. Maguire was selected as ACC Golfer of the Month for April 2017, which is the sixth time she has won the monthly award in her college golf career. On 11 May Maguire was voted the ACC Women’s Golfer of the Year for the second time in her career and was also selected for the All-ACC Women’s Golf Team for the third year in a row. On 11 May Maguire was selected for the College Sports Information Directors of America Academic All-District III Women’s At-Large Team, which recognises the nation’s top student-athletes for their combined performances athletically and in the classroom. On 23 May, Maguire was named the USA Women's Golf Coaches Association (WGCA) National Player of the Year for the second time in three years and became just the fifth player in history to win the award multiple times. She was also selected for the WGCA First Team All-Americans. Over 10 events in 2016–17, Maguire registered eight top-five placements, three victories, nine rounds in the 60s and 21 rounds of even or under par. She finished no lower than tied sixth over the course of the season including a Duke University and ACC all-time record 70.29 season stroke average, which is also the second lowest average in NCAA history after Lorena Ochoa's average of 70.13 in 2002. On 24 May Maguire was awarded the 2016-17 GolfStat Cup which is presented to the player with the best scoring average versus par with at least 20 full rounds played during a season. It was the second time in her career to win the cup. Maguire also finished in first place on the Golfweek/Sagarin women’s college end of season rankings. On 8 June Maguire won the Annika Award for the second time, becoming the only person to win it twice. She was also selected on the CoSIDA Academic All-America® Division I Women’s At-Large Team, only the second Duke golfer in history to be so. On 17 June Maguire won the British Ladies Amateur Golf Championship for the first time, beating Ainhoa Olarra of Spain 3 & 2 in the matchplay final held at the Pyle & Kenfig Golf Club, Waun-y-Mer, Wales. On 25 June Maguire was named the ACC Women's Golf Scholar-Athlete of the Year and was also named to the All-ACC Academic Team for the third year in a row. On 28 June Maguire won the Golfweek National Player of the Year award for the second time in her career and was also selected to Golfweek First Team All-America for the third straight season. On 6 July Maguire represented Great Britain & Ireland in the Vagliano Trophy against Europe, held at Circolo Golf Bogogno, Italy. Europe won 15 points to 9. Maguire contributed 1 point of the GB & Ireland total. On 5 July Maguire was selected for the Women’s Golf Coaches Association (WGCA) All-American Scholars Team for the third year in a row. On 10 July Maguire was awarded the 2017 Edith Cummings Munson Golf Award by the Women’s Golf Coaches Association (WGCA). She is only the second golfer in history to win both the Edith Cummings Munson Golf Award and the WGCA Player of the Year in the same season. On 6 August at the Kingsbarns Golf Links, Scotland, Maguire made the cut and was runner-up for the Smyth Salver awarded to the leading amateur at the Ricoh Women's British Open, one of the major professional championships, after rounds of 69, 71, 70, 75. On 15 August 2017, Maguire was awarded the Mark H. McCormack Medal given annually to the number one ranked female amateur golf player in the world. This is the third successive season Maguire has won the award thus equalling the all-time record holder Lydia Ko. On 12 September Maguire began the 2017-2018 College Golf season by winning the Jim West Challenge at The Fazio Course at The Club at Carlton Woods, The Woodlands, Texas, USA. Her rounds of 67, 63, 70 for a total of 200 included the lowest round of her career, a 63, which is the second lowest round in the history of Duke University, one stroke behind Liz Janangelo's 62. Maguire's 36-hole and 54-hole totals of 130 & 200 also broke the Duke all-time scoring records for two and three rounds. Her 13-under par total of 200 also broke the tournament all-time record. She also helped Duke win the team title at the tournament as well as setting a new school team scoring record of 33-under par total of 819. Duke also set a new Jim West Challenge tournament record for under par (-33) and 54-hole score (819). This is Maguire's fifth title of the calendar year 2017.
AmateurJunior Ryder Cup (representing Europe): 2008
Junior Solheim Cup (representing Europe): 2009, 2011
Vagliano Trophy (representing Great Britain & Ireland): 2009, 2011, 2015, 2017
Curtis Cup (representing Great Britain & Ireland): 2010, 2012 (winners), 2016 (winners)
Espirito Santo Trophy (representing Ireland): 2012, 2016