In the small town of Gaorah, Poland, Laib ben Kasreal was born, in 1841. His parents were Kasreal ben Laib, a shopkeeper, and his wife, Leah Joseph. Because of anti-Semitic persecution the family moved to England, where his father changed the family name from Kasreal to Caselberg, although they had no German connections. Young Laib took the personal name Myer. He became a travelling salesman in Wales.
In 1863 Caselberg emigrated to Southland, New Zealand. He spent two years in Invercargill and mining unsuccessfully for gold in Otago. After moving to Wellington in 1865, he heard of land being opened up in the Wairarapa Valley and walked over the Remutaka Range, carrying all his possessions. He settled in Featherston and, with 'Tiny' Pain of Martinborough, is reputed to have been the first to use the old Māori track for packing in retail goods by horse.
Caselberg travelled the isolated holdings of eastern Wairarapa, until then supplied by ship. He became friendly with a fellow Jew, Joseph Nathan, with whose help he set up a store in Featherston in 1865. Myer married Frances Marks in Wellington on 1 April 1868. The same year he shifted to Greytown, setting up another store. In 1874 he extended his operations to Masterton and in 1877 the mayor, R. G. Williams, laid the foundation stone of the business headquarters. Frances Caselberg died in Wellington in 1890, leaving Myer with five sons and two daughters. On 29 June 1892 he married Rosetta (Etta) Victoria Keesing in the Auckland Synagogue; they were to have three daughters.
In 1892 a new registered company, Wairarapa Farmers' Co-operative Association, was formed with a share capital of £100,000; it amalgamated Caselberg's businesses with Nathan's Tinui store. Caselberg was managing director of the new company until 1920. Three sons, Joseph, Alfred and David, later became managing directors of various departments.
By 1908 the WFCA was the largest inland trading organisation in the North Island. It had stores in Masterton, Carterton, Greytown, Featherston, Martinborough, Alfredton, Pahiatua and Dannevirke. By 1895 there was a butter factory at Kopuaranga and a cheese factory at Greytown. The company had a London address and the Wellington agents were Joseph Nathan and Company. In 1897 a four-storeyed brick building was erected in Lambton Quay, Wellington, and in Masterton the company opened a furniture factory and bought an upholstery business. Early in the 1900s garages, motor vehicle showrooms and workshops were opened in Masterton and Pahiatua and granaries in Masterton and Greytown. The company held agencies for Austin and Dodge vehicles. In its rapid growth during its first decades the company represented a rare example of successful commercial expansion beyond the region.
Energetic, bearded, deeply orthodox in religion, Caselberg was heavily involved in public affairs and local government. He was chairman of Greytown's school committee and the Waiōhine River Board, and treasurer of the hospital committee. He also represented Greytown on the first county council and he was a justice of the peace for many years. After moving to Masterton he was a member of the borough council for three years and was mayor from 1886 to 1888. During his mayoralty the municipal gasworks were established. He was chairman of the Masterton hospital committee and the school committee. In 1878 he was appointed to the Masterton Town Lands Trust.
Caselberg's Masterton store burned down in 1877 and the Greytown one in 1881. As a result he took great interest in fire control, and while visiting England on business in 1887 he negotiated the purchase of a Shand Mason steam-powered fire-pump. Frances Caselberg christened the engine 'Jubilee' after Queen Victoria's golden jubilee that year. The restored engine stands in front of Masterton Fire Brigade's building.
Anti-German feeling in 1914 brought a call for the family's internment as aliens. Caselberg publicly disclosed his non-German origins and drew many expressions of esteem and affection, which led to the call fading away. The brief controversy had no effect on the WFCA's business. Caselberg retired in 1920, and died on 23 June 1922 in Rosetta House, Masterton, which he had built in 1874. His body was taken to Wellington by special train for burial. Caselberg was survived by his wife, five sons and five daughters; Rosetta Caselberg died in 1941.