Whilst only 16, Staniforth began playing for the Sunderland first-team. In her first season in senior football, Staniforth was instrumental in helping Sunderland win the FA Women's Premier League Northern Division and reach the 2009 FA Women's Cup final, where they were beaten 2–1 by Arsenal. The following season, Staniforth figured prominently as a key player in Sunderland's success as they finished fifth in their first season back at the top level.
Upon Sunderland's failed bid to join the FA WSL in 2010, Staniforth joined successful applicant, Lincoln Ladies. She started every game in her first season at the club, netting three times. The most notable of her goals in her first season in the Women's Super League came in the away fixture at Doncaster Belles. Staniforth helped her side reach fourth place in the league.
In the 2012 season, Staniforth bettered her goalscoring tally substantially, scoring on a further 6 occasions in all competitions. In the two ultimate games of the season, she scored winning goals for Lincoln: an extravagant lob from 35 yards against Everton Ladies, and against Chelsea Ladies, lifting the side to fifth in the table.
In December 2012, it was announced that Staniforth would be joining Bristol Academy, against whom she had scored twice in the preceding 2012 FA WSL season. Staniforth scored her first goal for the club in the home fixture against Doncaster Rovers Belles, with a 22-yard drive. In her first season at the Stoke Gifford Stadium, Staniforth reached the second FA Women's Cup final of her career. During the match she suffered a serious knee injury and was stretchered off. After only a short time at the club, Staniforth subsequently spent the remainder of the season sidelined, with plans to return in time for the 2014 season. Bristol ended the season runners-up, losing 2–0 on the final day of the season to champions Liverpool.
In February 2014, Staniforth joined reigning FA WSL champions Liverpool for a reported five-figure fee. Staniforth sustained an injury during pre-season, this time to the anterior cruciate ligament in her other leg, which put her out of action for the entirety of the 2014 season. Despite not seeing any playing time because of injury, Staniforth's Liverpool went on to win the FA WSL title for the second consecutive year.
In January 2016 it was announced that Staniforth had left Liverpool and returned to Sunderland on a two-year contract.
At the beginning of the 2017 season, Lucy was awarded penalty-taking responsibilities for her club. In both the fifth and sixth round of the FA Women's Cup, against Aston Villa and Chelsea respectively, Staniforth scored from the penalty spot. She then scored on two further occasions in the league, from the spot against her old club Bristol City and directly from a corner against Yeovil Town with the outside of her foot.
Following Steph Bannon's departure from the club, Staniforth was named club captain of Sunderland in July 2017.
Staniforth featured for England in their fourth-place finish at the 2008 FIFA U-17 Women's World Cup in New Zealand, scoring an excellent goal against Japan in the quarter final.
In July 2009 Staniforth also took part in the 2009 UEFA Women's Under-19 Championship in Belarus, which England won. In her time with the U-19s, she scored twice. In 2010 Staniforth helped England reach the final of the 2010 UEFA Women's Under-19 Championship in FYR Macedonia, where they lost their title to France. Later that summer Staniforth played in two of England's games at the 2010 FIFA U-20 Women's World Cup in Germany.
Staniforth subsequently joined the ranks of the U-23s, making her debut against Norway in February 2012. Staniforth scored an impressive 25-yard drive in the first game of the Under-23 Four Nations La Manga Tournament, helping her team beat Sweden 2–0.
Staniforth comes from a footballing family, where both her father and brother were professionals. Gordon Staniforth, her father, played for York City and Hull City, his hometown club, as well as a host of other clubs. Her brother, Tom, also played for Sheffield Wednesday but died at the age of 20. Throughout her career Lucy has worn the number 37 — Tom's old Sheffield Wednesday squad number. After hitting the first goal in Lincoln's 3–0 WSL win at Doncaster Rovers Belles, Lucy revealed a T-shirt paying tribute to Tom on the tenth anniversary of his death.Liverpool Ladies
WSL Women's Super League: 2014
WSL Women's Super League: 2013, Runner-up
FA Women's Cup: 2013, Runner-up
FA Women's Cup: 2009, Runner-up