Lake broke the 29-year-old UK Under 17 high jump record, clearing 1.90m in 2013, and broke the 23-year-old UK Under 20 high jump record with 1.93m in 2014. She also broke the world indoor pentathlon youth (under-18) record in 2014, with a score of 4284 points. At the 2016 Olympic Games in Rio, she became the first British woman to reach an Olympic high jump final since Debbie Marti in 1992.
Born in Reading, Berkshire, England, Lake began to compete competitively as an under-13 in 2008 at Slough Juniors Athletics Club. In 2009, she broke the UK under-13 pentathlon record with 3,046 points. In 2011, she broke the under-15 record with 3,755 points. At the English Schools Championships, she won the pentathlon and long jump in 2010 (under-15) and the long jump in 2012 (under-17). In 2013, she finished second at the British Indoor Championships in the high jump.
Lake competes for Windsor, Slough, Eton and Hounslow Athletic Club and is coached by her father Eldon Lake. She also receives specialist coaching from Jeremy Tigar in the throw disciplines (shot put and javelin), Richard Aspden in the high jump and Ian Grant in the hurdles.
At the 2013 World Youth Championships in Donetsk, Lake led after day one of the heptathlon by 192 points. The day included a UK Under 17 record of 1.90 m in the high jump, a height that would have been good enough to win the individual high jump event. The previous record of 1.89 by Debbie Marti, had stood for 29 years. However, on day two she could only manage 4.63 metres in the long jump, followed by two fouls. Then in the javelin, she could only throw 30.81 metres. These performances saw her drop down to sixth and she withdrew from the competition before the final event, the 800 metres.
In February 2014, Lake competed in an indoor athletics event in Vaxjo, Sweden, where she broke the World Youth (U18) Record for the pentathlon, improving Carolina Klüft's score of 4261, with 4284 points.
On 2 and 3 May 2014, Lake won the prestigious Multistars international event in her debut at the senior heptathlon. Still 9 days short of her 17th birthday, she became by four years the youngest champion in the 27-year history of the event. Her score of 5896 points exceeded by 780 points the performance (5116 points) achieved by Olympic champion Jessica Ennis-Hill at the age of 17 when placing 13th in the same event in 2003. Lake's performance also exceeded by 415 points the previous UK Youth record for the heptathlon (5481 points), set in 2009 by Katarina Johnson-Thompson.
On 18 May, she broke the 23-year old UK junior high jump record with a clearance of 1.93 m in Loughborough. The previous record of 1.91 m was set by Lea Haggett in 1991 and was equalled in 1997 by Susan Jones.
On 31 May and 1 June 2014 Lake competed in the Hypo Meeting, generally considered the premier multi-events meeting in the annual athletics calendar. She achieved 17th place, improving her lifetime best score and UK Youth record by 185 points to a total of 6081 points. This performance also ranked her second on the UK all-time Under 20 list, behind Katarina Johnson-Thompson, and improved by 90 points the European Youth record of 5991 points achieved in 2005 by Tatyana Chernova of Russia, the 2011 World Champion at heptathlon. Lake's individual performances in Götzis included three new personal bests in the shot putt, a personal best in the 200 metres and two personal bests in the javelin throw.
In July 2014, Lake and her coaches initially planned to have her compete in the 2014 Commonwealth Games in Glasgow, but decided against it and headed for the 2014 World Junior Championships in Eugene, Oregon instead. She began her campaign at the championships on 22 July with the 100 m hurdles, finishing fourth in the third heat and 12th overall with a time of 14.29 s. She stormed to the top of the heptathlon table with 2,096 points, setting a British junior record of 1.94 m in the heptathlon high jump. The only athlete to have cleared 1.85 m, she attempted a world youth record and British senior record height of 1.97 m, but failed to clear it. She began the afternoon session with a win in the heptathlon shot put, throwing 14.17 m to take her total to 2,901 points. She closed the day with the 200 m, finishing in third place both in the third heat and overall behind German world youth champion Celina Leffler and Dutch athlete Nadine Visser to end with a total of 3,821 points and remain at the top of the heptathlon table.
Lake started the second day relatively poorly, finishing fourth in the heptathlon long jump in with a distance of 5.90 m. However, her total rose to 4,640 points, still enough to keep her at the top of the heptathlon table and in pole position for the gold medal. She finished in sixth place at the heptathlon javelin throw the same morning with a personal best of 41.66 m, but remained on top of the overall table with an additional 699 points to her total. Lake then closed the day with a seventh-place finish in the 800 m with a personal best time of 2:21.06, but secured the gold medal with a final total of 6,148 points, ahead of Cuban Yorgelis Rodríguez and Visser, who went on to win bronze in the individual 100 m hurdles event on 28 July.
On 27 July, the final day of the championships, Lake competed in the individual high jump event, and took her second gold medal with a height of 1.93 m. She and silver medallist Michaela Hrubá of the Czech Republic were the only competitors to have managed to clear 1.91 m. Lake went on to make three attempts at 1.97 m as she did in the heptathlon high jump five days earlier, but again failed to set a new British senior record.
On 18 August 2016, Lake competed in the Rio Olympics in the individual high jump event for Team GB, achieving a personal best of 1.94m and reaching the final. In the finals, Lake successfully completed a height of 1.93m coming joint 10th alongside Iryna Herashchenko and Inika McPherson.