Born in Dundee, Lewis Toshney joined the Celtic Youth Academy at the age of 12 and progressed through all of the age levels with the Glasgow club. While with Celtic, Lewis also represented Scotland at U17, U19 and U21 level. He signed a two year contract extension in 2011, after helping the Celtic Under 19 side to a league and cup double and made his professional debut in February that same year. In January 2012, Lewis moved on loan to Kilmarnock for the remainder of the 2011-12 season, where he played regularly as part of the first team set up at Rugby Park. The following season, he moved to Dens Park on a season long loan to join newly promoted Dundee. While with Dundee, Lewis scored his first senior goal having made 26 appearances. At the end of the season he returned to Celtic where he spent a year playing with the Development Squad.
Upon the expiration of Celtic contract in 2014, Lewis signed for Ross County on a short term contract, making his debut in a 2-1 defeat of Dundee. His time in Dingwall ended in January 2015 and the following month he signed for SPFL Championship side Cowdenbeath. Impressive form at Central Park during the remainder of the 2014-15 season saw him earn a move to rival Fife side Raith Rovers during the summer and he quickly became a mainstay at Starks Park under manager Ray McKinnon, helping Rovers reach the 2015-16 Premiership Play-offs.
The following season, Lewis followed Ray McKinnon to Tannadice, agreeing a three year deal with United for an undisclosed fee.