As part of a group of six submarines selected for Mediterranean service, UB-42 was broken into railcar sized components and shipped to Pola where she was assembled, launched and commissioned in March 1916.

[6] The German Imperial Navy ordered UB-42 from AG Weser of Bremen on 31 July 1915 as one of a series of six UB II boats (numbered from UB-42 to UB-47).

The U-boat could carry up to 28 tonnes (28 long tons) of diesel fuel, giving her a range of 7,030 nautical miles (13,020 km; 8,090 mi) at 5 knots (9.3 km/h; 5.8 mph).

[1] As one of six U-boats selected for service in the Mediterranean while under construction, UB-42 was broken into railcar-sized components and shipped overland to the Austro-Hungarian port of Pola.

UB-42 and three of her sister ships in the Pola Flotilla were ordered to Constantinople and, en route, had to navigate through the Dardanelles, which had been heavily mined by the Allies in the middle of 1916.

[18] On 14 April, Wernicke torpedoed the British Acacia-class sloop HMS Veronica 45 nautical miles (83 km; 52 mi) off Alexandria, damaging the 1,200 tons vessel.

[1] The 27-year-old Schwarz, who had previously commanded the Type UB I boat UB-14,[22] led UB-42 to sink her largest ship, Cestrian, on 24 June.

The 8,912 GRT Leyland Line steamship was in use as a troopship, carrying 800 troops and horses when Schwarz sank her 4 nautical miles (7.4 km; 4.6 mi) southeast of Skyros in the Aegean.

Three of Cestrian's crewmen were killed in the attack and, according to R. H. Gibson and Maurice Prendergast, "splendid discipline" among the embarked troops was the sole reason that none were lost.

[25] While remaining in the Black Sea, UB-42 sank the sailing ships Agios Georgios on 10 October, and Francesco Patrino in November.

[30] In September, Kapitänleutnant Hans Georg Lübbe (who had succeeded Herbert Nolde after his two-month stint as commander of UB-42) led the U-boat in sinking her final ship.

[11] After the signing of the Armistice of Mudros on 30 October ended the war for the Ottoman Empire, the four remaining U-boats of the Constantinople Flotilla—UB-14, UB-42, UC-23, and UC-37—fled to Sevastopol.