From the tenth century, shires, or counties such as Surrey, were divided into administrative areas called Hundreds. (In the Danelaw this division was called a Wapentake).
Mitcham was in the Wallington Hundred. Tooting was in the Brixton Hundred. The boundary between the two was the River Graveney.
With the formation of the County of London (1888) and then the Borough of Wandsworth (1899), parts of the parish of Tooting-Graveney were in Mitcham because of this river boundary.
So in 1903 the railway line was chosen as the new boundary and the area north of it went to Wandsworth Borough and the south of it to Mitcham.
After 1903, Tooting Junction Railway Station became part of Mitcham, as did Finborough Road and Tooting Old Hall.
Source: page 36 Mitcham Histories: 2 North Mitcham, by EN Montague.