Followed his father into the printing trade. He appears as a lithographic printer in Hamilton St, Greenock, 1834-61.

Shown in 1851 census as widower with no mention of children.

Best guess is that he married Agnes Bain in 1839, and after her death, married Christina.

Notes

John Johnston, lithographic printer, Greenock, to Agnes Bam, only surviving daughter of late Daniel Cumming of Port Glasgow at Greenock on 11th February 1839 by Reverend Campbell (Greenock Advertiser 12.2.1839). Looks like a first marriage. Ancestry.com has JJ b. 16 Dec 1807 who married Agnes Bain Cumming on 11 Feb 1839.
But note also: John Johnston, lithographer, Greenock to Catherine, daughter of late Colin White, Dunoon, at Glasgow on 12th June 1854 (Greenock Advertiser 16.6.1854). This could be a second marriage.
Poss death: John Johnstone, former lithographic printer, died at 9 Argyle Street, Greenock on 7th June 1880. (Greenock Telegraph 7.6.1880).