He was born in Halifax, Nova Scotia, the eldest surviving son of Captain John Willshire by Mary, daughter of William Linden of Dublin.
It joined the Royal Regiment and the 9th Foot in the 5th (Leith's) division three days before the Battle of Salamanca, where Willshire was twice wounded.
He commanded the light companies of the brigade at the passage of the Bidasoa, the Battle of the Nive (9–11 December) and the repulse of the sortie from Bayonne (14 April 1814).
In 1815 his battalion was sent to the Netherlands, but was too late for the Battle of Waterloo and instead went on to Paris, where Willshire was employed for a short time on the staff.
A quarrel between the chiefs, in which the British authorities intervened, led to a well-planned attack on Grahamstown by Mokana with six thousand Kaffirs on 22 April.
On 30 August 1827 he was made lieutenant colonel without purchase of the 2nd (Queen's), stationed at Poona, serving with that regiment for nearly ten years.
He was made a Knight Commander of the Order of the Bath in 1839 and created a baronet (Willshire of the East Indies) for the capture of Kelat.