His family seat was Abington Hall in Abington Magna outside Cambridge. In 1778 Mortlock bought himself the Freedom of Cambridge for £40. Having originally followed his father as a woolen merchant he founded in 1780 the first bank in Cambridge, later one of the banks that founded Barclays Bank. and:"his bitterest political opponents never impeached his business honesty".
In 1792 he was appointed to the lucrative post of Receiver General of the Post Office, a post he held until 1806.
He died in 1816. His great-grandson, also John, was knighted. A grandson Frederick William Mortlock worked in customs in Jamaica.
Mortlock, who was a great friend and supporter of Pitt the Younger, was called corrupt by his political opponents, though as the plaque suggests he had a different view of things:"without influence, which you call corruption, men will not be induced to support government, though they generally approve of its measures".
16th Parliament of Great Britain]
In 1782 he became an alderman, then mayor and, in 1784, the Member of Parliament for Cambridge.