Bishop Lambert left Marseilles on 26 November 1660 accompanied by Fathers De Bourges and Deydier,[6] and reached Mergui in Siam 18 months later. Bishop Pallu joined Bishop Lambert in the capital of Siam, Ayutthaya, after 24 months overland, but Bishop Cotolendi died upon arrival in India on 6 August 1662. Siam thus was the first country to receive the evangelization efforts of the Paris Foreign Missions Society, to be followed by new missions later in Cochinchina, Tonkin and parts of China.[5]
In 1664, Bishop Lambert together with Bishop Pallu presided a synod in Ayutthaya.[7] They founded in 1665-66 the general seminary in Ayutthaya, Siam[8] (the Seminary of Saint Joseph[9] then Seminary of the Holy Angels, at the origin of the College General now in Penang, Malaysia).
In 1669, Bishop Lambert went to Tonkin together with the secular priests Jacques de Bourges and Gabriel Bouchard to establish a church there, and created the congregation of the Lovers of the Holy Cross (Amantes de la Croix de Jésus-Christ) in 1670.[10]
Bishop Lambert chiefly worked in Siam; he went to Cochinchina in two occasions: from September 1671 to March 1672, and from September 1675 to March 1676.[7]
He died in 1679, in the capital Ayutthaya of Siam.