After his return from the Baltic in November 1812, Saumarez did not serve again afloat. Promotion up the ranks of admirals followed as those ahead of him in seniority died. He flew his flag again as Admiral of the White from April 1824 till 10 May 1827 as Port Admiral, Plymouth, and in 1830 he became Admiral of the Red, the highest rank short of Admiral of the Fleet. He received an Honorary DCL from Oxford in 1814, but it was not until 1 October 1831 that the longed-for reward was announced – his being raised to the peerage as Baron de Saumarez. He was the first native-born Guernseyman so honoured and the island went wild with excitement, greeting his return from England on 25 October, after a passage beset by gales, with cheering crowds and, the next day, a more formal civic address. His journeys to England thereafter were infrequent and his only attendance in the Lords was to vote for the Reform Bill.

Although retired from active naval service other than his time at Plymouth, he became heavily involved in a wide range of other public and charitable services, particularly those concerned with the Church, the Bible and schools. After three years of declining health, he died on 9 October 1836, aged 79. He was buried at his parish church in Castel at what was intended as a private ceremony, but over 1,000 people are said to have attended, all the shops in town closed and minute guns fired from Fort George throughout the day. Sadly, four of his eight children died before him, two in infancy, but he had 84 nephews and nieces and their children living, to each of whom he left £100. He was succeeded as second Baron by his eldest son James, Rector of Huggate in Yorkshire. Martha lived on for 13 years and was buried beside him in 1849. In 1876, a 99-foot-high obelisk in Guernsey granite was erected in his honour on top of his favourite hill in Delancey Park. It was demolished by the German occupation forces in 1943 and a group was set up in 2006 to study the possibility of its replacement on the remaining base of the old one to a new design.