An ablation catheter includes a flexible elongated shaft having a flexible electrode at its distal end. The flexible electrode includes an inner, flexible, electrically-conductive element and an outer, flexible, electrically-conductive polymer layer in electrical contact with the inner, flexible, electrically-conductive element. The catheter further includes an electrically insulative sheath surrounding at least a portion of the flexible electrode and a linear displacement mechanism capable of adjusting at least one of the electrically insulative sheath and the flexible electrode to vary a length of the flexible electrode exposed at the distal end of the shaft. Also disclosed is an adjustable length ablation electrode and methods of ablating tissue.