He became physician to Queen Elizabeth I, who in 1581 sent him, at Tsar Ivan IV's request, to the Russian court, where he attended the tsarina, and acquired a reputation which still survives.
Jacob recommended Lady Mary Hastings to the tsar for his seventh wife.
Happily for the lady, the tsar died before the conclusion of the negotiations, which were opened in 1583 with the sanction of Elizabeth.
Jacob returned to England with Sir Jerome Bowes, the English envoy in Russia, about March 1584.
On 21 May 1583, he was admitted a licentiate of the Royal College of Physicians in London, a candidate on 12 November 1585, and a fellow on 15 March 1586.