Eric Montague said in his book Mitcham Histories : 12 Church Street and Whitford Lane, page 107, that Mitcham’s post office had occupied 5 locations, as listed below:
1st : at Westhall’s grocery shop in the Broadway
The 1855 directory shows the postal services available:
It lists Joseph WESTALL as grocer and cheesemonger as well as the post office receiving house in Lower Mitcham.
2nd : in a small shop near Mitcham Station
This photo from 1895 shows part of the words ‘Post Office’ above the shop.

clip from Merton Memories, photo reference Mit_Public_Services_18-2, copyright London Borough of Merton
3rd : a purpose built building in the Broadway
The words ‘Post Office’ can be seen etched in the windows on the building on the right in this photo of around 1910:
According to Eric Montague in his book Mitcham Histories : 4 Lower Mitcham, pages 127-8, this post office was
erected in about 1900 … a three-storeyed building … its rather fussy facade including false timber framing to simulate an Elizabethan structure.”
4th : Post Office and Telephone Exchange building on the corner of London Road and Elmwood Road
Built around 1920, shown here in this 1953 photo:

clip from Merton Memories photo, reference Mit_Streets_Lon_38-46, copyright London Borough of Merton
5th : Langdale Parade
In 1961 the post office moved to Langdale Parade in the Fair Green. The telephone exchange building remains.
Currently, in 2019, the Langdale Parade post office has moved to a smaller shop nearby at number 5: