One of the services he provided was procuring a mistress for Leopold: he prevailed upon one of his cousins, Caroline “Lina” Philippine Auguste Bauer, an actress, who went through a form of marriage with Leopold that was almost certainly invalid. (Her marriage contract specified that she was to receive the title Countess of Montgomery).
When Leopold came under scrutiny for the position of King of the Belgians, Stockmar's cousin was quickly discarded, and Leopold was made king in 1831. Stockmar took up residence in Coburg, continuing to advise Leopold. In 1837, he was sent by Leopold to serve as advisor to Queen Victoria: one of his first tasks was to brief her on whether Leopold's nephew, Prince Albert (later Prince Consort), was a suitable mate. After the marriage of Victoria and Albert, Stockmar became their unofficial counsellor, and intervened in several crises.
His prominence in the United Kingdom's political circles led to resentment at what was seen to be Albert's (and, generally, German) intervention in the UK's affairs.
Stockmar was raised to the rank of baron by the King of Saxony.