The choir, of five bays, was built between 1283 and 1315 to the design of Richard Lenginour,  and is an early example of Decorated Gothic architecture. The piers have strongly modelled attached shafts, supporting deeply moulded arches. There is a triforium gallery with four cusped arches to each bay. The sexpartite vault , which is a 19th-century restoration, is supported by clusters of three shafts which spring from energetic figurative corbels . The overall effect is robust, and contrasts with the delicacy of the pinnacled choir stalls, the tracery of the windows and the rich decoration of the vault which was carried out by the ecclesiastical designers, Clayton and Bell .  The choir stalls, dating from about 1380, are one of the glories of the cathedral. 
The most recent renovation of the Cathedral, begun in 2002 under the direction of Bishop Robert C. Morlino, continues under Bishop George Leo Thomas. Both the upper and lower levels of the Cathedral were renovated. The lower level was completely remodeled to include two new social halls and a complete kitchen. The lower level was named in honor of the first bishop of Helena, John B. Brondel . The upper-level renovation included the sanctuary, Saint Joseph's Chapel, new baptistry, and the installation of the elevator in the north transept for improved access between levels.