Most of the interior of the northern half of the island is taken up by a large granite batholith that was created by substantial volcanic activity around 60 million years ago in the Tertiary period. There is an older outer ring of coarse granites and an inner core of finer grained material. Sedimentary rocks dominate the southern half of the island, especially Old and New Red Sandstone. Some of these sandstones contain fulgurites - pitted marks that may have been created by Permian lightening strikes. Sand dunes are preserved in Permian sandstones near Brodick, there are localised outcrops of Triassic rocks and even a rare example of Cretaceous chalk.

Visiting in 1787, the geologist James Hutton found his first example of an unconformity to the north of Newton Point near Lochranza, which provided evidence for his Plutonist theories of uniformitarianism and about the age of the Earth. This spot is one of the most famous places in the study of geology.

The Pleistocene glaciations almost entirely covered Scotland in ice and Arran's highest peaks may have been nunataks at this time. After the last retreat of the ice at the close of the Pleistocene epoch sea levels were up to 70 metres (230 ft) lower than at present and it is likely that circa 14,000 BP the island was connected to mainland Scotland. Sea level changes and the isostatic rise of land makes charting post-glacial coastlines a complex task, but it is evident that the island is ringed by post glacial raised beaches. King's Cave on the south west coast is an example of an emergent landform on such a raised beach. This cave, which is over 30.5 metres (100 ft) long and up to 15.3 metres (50 ft) high, lies well above the present day sea level. There are tall sea cliffs to the north east including large rock slides under the heights of Torr Reamhar and at Scriden (An Scriodan) at the far north end of the island.

Arran has a number of villages that are mainly found around the shoreline. The "capital" is Brodick (Old Norse: "broad bay"), which is the site of the ferry terminal, several hotels and the majority of shops, although Lamlash is actually the largest village on the island. (In 2001 the former's population was 621 and Lamlash's was 1,010.) Brodick Castle is a seat of the Dukes of Hamilton. Other villages include Lochranza in the north, Corrie in the north east, Blackwaterfoot in the south west, Kildonan in the south and Whiting Bay in the south east.

Arran has three smaller satellite islands: Holy Isle lies to the east opposite Lamlash, Pladda is located off Arran's south coast and tiny Hamilton Isle lies just off Clauchlands Point 1.2 kilometres (0.75 mi) north of Holy Isle. Eilean na h-Àirde Bàine off the south west of Arran at Corriecravie is a skerry connected to Arran at low tide.

Other islands in the Firth of Clyde include Bute, Great Cumbrae and Inchmarnock.

The influence of the Atlantic Ocean and the Gulf Stream create a mild oceanic climate. Temperatures are generally cool, averaging about 6 °C (43 °F) in January and 14 °C (57 °F) in July at sea level.

The southern half of the island, being less mountainous has a more favourable climate than the northern half and the east coast is more sheltered from the prevailing winds than the west and south.