He played for Western Australia at the 2003 and 2004 Under-18 Championships, and was named in the 2004 Under-18 All-Australian team. He played colts football for West Perth in the WAFL in 2003 and 2004, and was named as a half-forward flank in the Team of the Year in both seasons. LeCras made his senior debut for West Perth against Claremont in round 20 of the 2004 season, and played two further games for the season, in rounds 21 and 23.
LeCras was recruited by the West Coast Eagles with the 37th pick overall in the 2004 National Draft. After spending the first half of the season with West Perth, He made his debut for West Coast in round 10 of the 2005 AFL season, kicking two goals and recording 12 disposals. He played one more game before being dropped back to the WAFL, where he played the rest of the season.
He was recalled in 2006 for the Round 14 clash against Hawthorn, but only managed a total of 16 disposals in two games before being dropped and recalled for the Round 22 demolition of Richmond, where LeCras put in a surprise performance, booting 5 goals. LeCras placed fourth in the Sandover medal for the best player in the WAFL in 2006. The award was won by fellow West Coast Eagle player Matt Priddis. He fought his way back into the team in 2007, and had a stellar season, kicking 36 goals for the season.
In 2009, Mark LeCras kicked 58 goals in the home-and-away season, leaving him in All-Australian contention. Ultimately he missed out on selection. In round 13, 2009, he scored his 100th AFL career goal in West Coast's 20 point win against Hawthorn at Subiaco Oval. For the overall 2009 season, LeCras became the most potent forward in the team kicking 30 goals in the first 12 rounds, and 58 for the season, topping the Eagles' goalkicking list.
In round 16 of 2010, LeCras kicked a personal best of 12 goals, doubling his previous record of 6, in an outstanding individual display against Essendon at Etihad Stadium. It broke the previous record of 11 (kicked by Stephen Milne) for the most goals kicked by an individual in a game at the stadium. This was one of a few highlights in a dark season for the Eagles in which they won the wooden spoon for the first time in its history. The performance also earnt LeCras 226 points in the Herald Sun Supercoach competition, the highest score for the 2010 season. LeCras later finished Third in the Coleman Medal with 63 goals from 21 games, behind Jack Riewoldt and Barry Hall. On 10 September 2010, LeCras was awarded the prestigious award of the West Coast Eagles Club Champion. On 13 September, LeCras was selected at forward-pocket in the 2010 All-Australian team, the only player from the West Coast Eagles in the team. He was also nominated as one of the three contenders for goal of the year for his effort against Essendon in Round 16, one of twelve goals scored on the night, eventually losing out to Lance Franklin for the award.
LeCras married Emily Marshall in a private ceremony on 23 December 2011, with the reception held at Cottesloe Beach. In February 2013, Mark and wife Emily welcomed their first child, a baby girl called Molly.