Sinclair attended the baptism of Prince Henry at Stirling Castle in 1594 and James VI gave him a "target" or hat badge worth 80 French crowns.
Sinclair told a Scottish merchant that Christian IV would visit Scotland and Anne of Denmark should prepare a lodging for him in Kinloch's house near the gates of Holyrood Palace.
Sinclair informed James VI that Christian IV of Denmark and other German princes would visit Scotland in the spring, and the queen's brother, the Duke of Holstein would come through and France and England and meet up.
[9] From Frederiksborg Sinclair sent James VI notice of the coronation of Christian IV in 1596 in advance of a formal invitation by the diplomat Steen Bille, suggesting that if he could not come in person, the Duke of Lennox and two other noblemen would be suitable.
The English ambassador Robert Bowes sent a copy of Sinclair's letter to William Cecil, suggesting the Earl of Crawford or Lord Sanquhar might go with the duke.
[16] On 20 April he wrote to the Earl of Salisbury thanking him for sending some money for him from King James, and asked for an annual pension with a patent letter written in Latin.
[22] Christian IV aggressive policy towards Sweden was met with reluctance and skepticism in Skåne, since the province would be the first to feel the impact of war in case of hostilities.
The King needed faithful supporters in the province, and hence Sinclair was made colonel of the Scanian Regiment in 1615, and became recipient of a number of land donations.
In 1617 Sinclair became a member of the privy council of the Danish realm; the King needed loyal followers in that august body, as its resistance towards his foreign policy was mounting.
Sinclair was the Danish King's envoy in Britain a number of times between 1606 and 1621, managing to influence James VI into adopting a more pro-Danish, and less pro-Swedish, stance.