Simpson – who came through the ranks at Manchester United – was found throttling Stephanie Ward when police arrived at her house following a 999 call in December 2014.
The court heard that he had to be dragged away by an officer while she was left crying hysterically and cowering in the corner; Simpson – who pleaded not guilty – will be sentenced next month.
Simpson, 28, was handed his United debut by Sir Alex Ferguson on a pre-season tour against Kaiser Chiefs in 2006.
However, he failed to break into the first team at Old Trafford and – after a loan spell – he joined Newcastle United permanently in 2010 and went on to make 115 appearances for the club.
He moved to QPR in 2013 before linking up with Leicester after their promotion to the Premier League last summer; he made 16 appearances under Nigel Pearson, the last of which came in a 2-0 defeat at Manchester City on March 4.
Ward has issued a statement saying she is upset at the outcome of the trial and did not support the prosecution.