After the resignation of Richard Cromwell Lockhart was continued by the Commonwealth ambassador in France. He took part as the English plenipotentiary in the negotiations which resulted in the treaty of the Pyrenees, and immediately on its conclusion went to England, where he had an interview with George Monck. Monck assured him that he intended to support the Commonwealth, and Lockhart accordingly refused to permit Charles II to come to Dunkirk. He also, according to Clarendon, turned down French inducements to hand over Dunkirk.