On the death of his mother in 1746, he inherited her property at Monks Horton in Kent (also known as Mount Morris), and took the surname Morris. He was MP for Canterbury from 1747-1761. He never married, and became increasingly eccentric in his later years: he grew his beard to unfashionable length, swam in the sea in all weathers and ate a restricted diet, reputed to include raw meat. He lived as a recluse on his Kent property, with occasional visits to take the waters at Aix-en-Provence for a stomach complaint.

In 1794, at the age of 81, he inherited the Irish title of 2nd Baron Rokeby on the death of his father’s second cousin, Revd. Richard Robinson. On his death, his titles and property passed to Morris Robinson (1757-1829), who took the surname Morris.

The most extensive (though incomplete and partial) description of Matthew Robinson was published by Samuel Egerton Brydges, who married his niece Mary Robinson, younger daughter of his brother William Robinson (Brief Character of Matthew Lord Rokeby, Lee Priory Press, 1817.