By his knowledge of copyhold law he secured a perpetual retainer from the lord of the manor of Taunton Dean, Somerset, whose rights were the subject of continual litigation.
Following the passing of the Reform Act in 1832 Manning was appointed as a revising barrister in which office he served the Borough of Newport in the Isle of Wight.
He was raised to the degree of a serjeant-at-law on 19 February 1840, received a patent of precedence in April 1845, and was made Queen's ancient sergeant in 1846.
This dignity, revived at his own suggestion after a long interval of dormancy, entitled him to a seat in the House of Lords ex officio, but gave him no right of speaking, unless consulted, or of voting.
He was twice married: first, on 7 September 1820, to Clarissa, daughter of William Palmer of Kimbolton, Herefordshire (died 15 December 1847, aged 51); and secondly, on 3 December 1857, to Charlotte, daughter of Isaac Solly of Leyton, Essex, and widow of William Speir, M.D., of Calcutta (died 1 April 1871).