Mr. Henderson was one of those ministers who went with the Scots army to England in the year 1640, every regiment having one of the most able ministers in the bounds where they were raised as chaplain, and when the treaty was set on foot which began at Rippon, and ended at London, he was also one nominated as commissioner for the church, the duties of which he discharged with great prudence and advantage, and the very next year, he was, by the commission of the general assembly, authorized to go with lord Loudon, Warriston and Barclay, to the king, to importune him to call his English parliament, as the only and best expedient to obtain an honourable and lasting peace; but his embassy had not the desired effect. After his return, he was chosen moderator to the general assembly _anno_ 1643, and when the English commissioners, _viz._ Sir William Armyn, Sir Harry Vane the younger, Mr. Hatcher and Mr. Darly from the parliament, and two ministers, Mr. Stephen Marshal a presbyterian, and Philip Nye an independent, from the general assembly of divines at Edinburgh, where the general assembly of the church of Scotland was then fitting, craving their aid and counsel upon such an emergent occasion, he was among the first of those nominated as commissioners to go up to the parliament and assembly of England. And so in a little after, Mr. Henderson and Mr. Gillespie, with Mr. Hatcher and Mr. Nye, set out for London to get the solemn league ratified there (the rest of the commissioners staying behind until it should be returned). Upon their arrival at London, and having received a warrant from the parliament to sit in the next assembly (which warrant was presented by Mr. Henderson), the assembly sent out three of their number to introduce them; at their entry Dr. Twisse the prolocutor welcomed them unto the assembly, and complimented them for the hazard they had undergone on their account both by sea and land, in such a rigorous season (it being then November); after which they were led to a place the most convenient in the house, which they kept ever after[64]. Again in the year 1646, being sent down from London to attend the king, then with the Scots army at Newcastle, at which time the general Assembly appointed also Messrs. Robert Blair, James Guthrie, Robert Douglas, and Andrew Cant, to wait on his majesty; here Mr. Henderson officiated for some time as his chaplain; and although he and Mr. Blair, of all the presbyterians were the best beloved of the king, yet they could by no means prevail upon him to grant the first demand of his subjects, yea, he obstinately refused, though they besought him on their knees. In the interval of these affairs, a series of letters was continued betwixt the king, assisted by Sir Robert Murray on the one hand, and Mr. Henderson on the other; the one in defence of Episcopacy, and the other of Presbytery, which were exchanged from the 10th of May to the midst of July as each person was in readiness.