Coming from an impressive karting career that included wins at several high-profile events, Merhi debuted in the 2006 Formula Renault Italy Winter Series to take third place. In 2007 he raced in the Formula Renault Eurocup while also getting a taste of GT Racing and F3 in his native Spain. He finished second in the 2008 Formula Renault WEC series and also ran in the Eurocup, also accompanied by more spot F3 appearances. After joining the F3 Euro Series in 2009, he reached PREMA in 2011 to win the Euro Series and the FIA F3 International Trophy. 2012 saw him switch to DTM, while in 2014, he came third in the Formula Renault 3.5 series. In 2015, he eventually made his Formula 1 debut with Marussia. In recent years, he ran in FIA Formula 2, and also focused on LMP2 endurance.