The son of John and Eliza Mitchell, he was educated at a private academy and apprenticed as a cabinet maker.
In 1887 Sagar and his father John founded the firm of S. & J. Mitchell, a photographic apparatus manufacturing and dealing business.
Then in 1994, during building alteration work at the premises in Northgate, three large sealed steel drums were discovered in the basement.
On examination, the drums were discovered to contain the original nitrate negatives of 800 Mitchell and Kenyon films in a remarkably good state of preservation.
In 2014 The Life and Times of Mitchell and Kenyon was produced at The Dukes, Lancaster and the Oldham Coliseum, with video elements by imitating the dog.