Taking up the law, Pearson joined the Inner Temple and was called to the Bar in 1924, where he was a Yarborough Anderson exhibitioner. He was a pupil of Walter Monckton (later the Viscount Monckton of Brenchley) in the chambers of Frederick Temple Barrington-Ward KC. He later joined the chambers of Sir William Jowitt KC where he began to build up a substantial common law practice. When Jowitt became Attorney-General, Pearson was briefed as his junior in many "derating cases" arising from the Rating and Valuation (Apportionment) Act 1928.