Trained as a military engineer, during the war Ellice served largely in North Wales, which was strategically important due to ports giving access to Ireland.

[3] From his uncle, the lawyer and antiquary Peter Ellice, Robert inherited the house and estate of Gwasnewydd near Wrexham, later rebuilt by his son and renamed Croesnewydd.

Myddelton being absent sitting in the Long Parliament, Ellice took Chirk with minimal resistance, taking the estate steward Watkin Kyffin prisoner.

A Royalist newspaper claimed that Ellice "plyed the Rebells so close with shot of all sorts, that in two Howres they took the House": the Parliamentarian news-sheet Mercurius Britannicus commented that "The truth is, Ellis had rather be doing anything on a Sunday than serving God".

He was mentioned by Archbishop Williams of York, a native of Conwy, as "but a weak piece, a potter [...] not to be relied upon" and as unable to stand up to the town's military governor, Sir John Owen of Clenennau.

[4] Ellice returned home after the war ended: like many other Royalist officers he was fined, though the relatively small sum of £200, reduced on appeal to £150 shows he was not a wealthy man.