The career of “Magistro Jacobo” began with the construction for
Peter II, Count of Savoy of
Yverdon-les-Bains Castle. At Yverdon, James was at first working with his father John, but by 1265 he is recorded as working alone, likely indicating the death of his father. From his apprenticeship to his father he went on to work under the guidance of an engineer from
Gascony, who had previously worked for
Henry III of England, Jean de Mézos. James worked at the at
Salins-les-Thermes at works dictated by Mézos in 1267–68. Following the death of
Peter II, Count of Savoy in 1268 he went on to work for his successor,
Philip I, Count of Savoy. Unlike Peter, who had preferred
Chillon Castle as his main residence, Philip preferred the
Viennois which was closer to his former see of
Lyon. Philip began construction of a new palace castle at . Building the castle would be Magistro Jacobo; his name "Saint George", only acquired after moving to England, may well be a reference to this castle of and/or his prior residence at the village of the same name. The first reference to this name in English records is Magistri Jacobi Di Sancto Georgio on 8 November 1280, two years after his arrival into England. His patron, King Edward I, probably met Master James of St George whilst returning from Crusade and visiting
Savoy in 1273. It was 25 June 1273 that King
Edward I of England visited so that his great-uncle
Philip I, Count of Savoy might pay homage to him in fulfilment of an earlier 1246 treaty whereby the castles of
Bard,
Avigliana, the palace of
Susa and town of
Saint-Maurice had been
enfeoffed to the
King of England. and
Conwy town walls at
La Bâtiaz windows dimensionally match those at
Harlech Castle James was responsible for the castles constructed for
Philip I, Count of Savoy in the between 1270 and 1275 at , , and . It is very possible that the simultaneous construction of these castles, three round tower castles and one octagonal tower palace castle, influenced Edward's decision to hire him to construct the castles in north Wales. Perhaps his last work in Savoy was at in the
Aosta Valley in the summer of 1275. There is an archival gap of the career of Magistro Jacobo between 1275 when he is last recorded in Savoy and 1278 when first recorded in Britain. Marshall has suggested that during the 'gap' he may have been working for the family of
Otto de Grandson who was close to both
Edward I of England and the comital family of
Savoy, possibly including the construction of
Grandson Castle and
Lucens Castle. Historian and author
A. J. Taylor proposed in 1950 that and Master James of Saint George were one and the same man. Taylor travelled from
Wales to
Savoy, noting the similarities of their castles' architectural features, citing the
garderobes at
La Bâtiaz Castle, the windows at
Chillon Castle along with the town walls at
Saillon as examples. ==British works==