Running between Croydon and Brixton station, it is operated by Transport UK London Bus.

Route 109 commenced operating on 8 April 1951 as a circular Purley - West Croydon station - Streatham - Elephant & Castle - Victoria Embankment - Westminster - Kennington - Norbury - Croydon - Purley route operating in both directions.

It replaced tram routes 16 and 18, operating from Brixton and Thornton Heath garages with AEC Regent III RTs.

On 23 January 1966, the Sunday service was withdrawn with the exception of a few early morning journeys operated by Thornton Heath garage.

Upon being re-tendered, Arriva London successfully retained with new contracts commencing on 1 February 2003 and 30 January 2010.