John Patrick Crichton-Stuart succeeded his father as third Marquess of Bute when he was just six months old. His mother died in 1859, and he became a Ward in Chancery in 1861; this made him a ward of the Chancery Court until he came of age, in order to protect his inheritance
The long minority of the Marquess led to difficulties with the further development of the Bute Docks. Although the existing dock provision was proving inadequate even by the late 1840s, his trustees were unwilling to commit the large amounts of capital investment required to expand the docks. In 1852 the trustees committed to the construction of a new dock, the East Dock, plus locks, basins and canals. These were completed in 1859.
Even this addition was insufficient, and in the 1860s Parliamentary permission was requested for further dock provision. Initially Parliament refused to sanction the estimated £1.2 million cost on the estate of a minor. In 1866 permission to build a new basin was granted but again, permission to build a new dock refused on the grounds that the third Marquess should reach his majority before the estate committed to such a large expenditure.
In 1868, Bute came of age and acquired control of his Scottish and Welsh estates. Construction of the Roath Basin commenced in this year and was completed by 1874. Parliamentary permission was granted in the same year to build a new dock adjoining the new Roath Basin, although delays in commencing work meant that this was not opened until 1887.
Bute had very strong links with Cardiff, serving twice as mayor in the 1890s. He was also President of University College, Cardiff. He was a great patron of architecture, and is particularly known for the collaboration with William Burges which resulted in the rebuilding of Cardiff Castle and the recreation of Castell Coch, creating two of the masterpieces of the Gothic revival.