Born in Paris, Mendy started his career at French club FC Versailles. Before trying his career in the Czech 1st Division in 2003 when he moved to FK Marila Příbram. The first club he played for in the Gambrinus Liga was FK Marila Příbram where he scored 16 league goals in 57 appearances before moving to FK Siad Most.
In 2007, he joined FK Mladá Boleslav on loan. Mendy made his European debut for Boleslaw against Palermo. In 2007, Mendy scored for Boleslav in the group stages of the UEFA Cup against Villareal CF and IF Elfsborg, However they failed to qualify past the group stage after losing to AEK Athens and Fiorentina. Despite only scoring one goal in 24 league appearances, his move to FK Mladá Boleslav was made permanent in 2008.
During the 2010–11 season, Mendy played all 30 league games for FK Mladá Boleslav scoring six goals as he helped them finish in fifth place in the Gambrinus Liga and also win the Czech Cup for the first time in Boleslav's history after beating SK Sigma Olomouc on penalties, Mendy scored one of the penalties for Mladá Boleslav in the final. The Cup win also saw Boleslav qualify for the UEFA Europa League for the following season. After his contract with FK Mladá Boleslav expired in June 2011, Mendy sought pastures new and became a free agent. He has found the back of the net 24 times in 206 appearances in the Gambrinus Liga.
On 19 July 2011, he played as an unnamed trialist on the right wing for Leeds United during a friendly match against Rochdale along with fellow trialists Jlloyd Samuel and Isiah Osbourne. He played for Leeds in the friendly against Sheffield Wednesday where he was officially confirmed as the unnamed trialist by his real name. With Mendy impressing in his first two pre season games with Leeds his trial was extended even further for the match against Norwegian club Sandefjord Fotball. With Mendy again impressing, starting his third pre-season match in a row for Leeds.
Mendy again played for Leeds in their 3–2 win against Newcastle United at Elland Road, coming on as a second-half substitute where he impressed on the right wing. After the match manager Simon Grayson said he would make a decision whether to sign Mendy and fellow trialist Boldi Bodor in the next two days after saying he felt both trialists had done ok in their spells. On 4 August 2011, Grayson confirmed on Yorkshire Radio the club were looking into signing Mendy permanently, however the deal was taking time as there were complications. Grayson confirmed that he was looking to sign the player and send him out on loan to help him learn the English game.
With Leeds enterring contract talks with Mendy, the move was called off on 11 August, after Leeds decided that they had to prioritise the transfer budget to sign players in other positions.
On 17 August 2011, Mendy signed for Chesterfield on a 1-year deal until 30 June 2012.
Mendy scored his first goal for Chesterfield in the League Two 3–0 victory over Bournemouth on 10 September 2011.Mendy was part of the side that won the Football League Trophy in March 2012 at Wembley Stadium, it was his cross that set up the first goal for Chesterfield. He was released by the club at the end of the 2011–12 season.
Mendy signed a two-year contract with German Club Hansa Rostock on 10 June 2012.
In 2014, he signed a contract with 1.FC Saarbrücken till 2016. He is gonna play with shirtnumber 4.
Alexandre is the older brother of Vincent Mendy who plays as a defensive midfielder.
FK Mladá Boleslav2011 Czech Cup Winner
Chesterfield2012 Football League Trophy Winner