Lars Christensen was born at Sandar in Vestfold, Norway. Born into a wealthy family, Christensen inherited his whaling fleet from his father, Christen Christensen. After completing middle school in 1899, he received training in Germany and at Newcastle followed by trade college in Kristiania (now Oslo). He started his career as a ship owner in 1906. He ventured into the whaling industry in 1909, and directed several companies, including Framnæs Mekaniske Værksted, AS Thor Dahl, AS Odd, AS Ørnen, AS Thorsholm and Bryde og Dahls Hvalfangstselskap.
Christensen was Danish consul in Sandefjord from 1909. In 1910 Lars Christensen had married Ingrid Dahl (1891–1976), daughter of wholesale merchant and ship owner Thor Dahl (1862–1920). He would later assume control of large part of his father's and his father-in-law's extensive businesses following their deaths during the 1920s.
Endurance, the ship that became famous after Sir Ernest Shackleton's failed Imperial Trans-Antarctic Expedition of 1914, was originally built for Christensen, who intended to use her for Arctic cruises for tourists to hunt polar bears. When this did not happen, Christensen sold the ship to Shackleton.
Christensen had a deep interest in Antarctica and its animal life. He was particularly interested in making geographical discoveries, and gave his captains wide latitude to do so. He financed several expeditions specifically devoted to the exploration of the Antarctic continent and its waters, and participated in some of these himself, even bringing his wife Ingrid with him in the 1936–1937 expedition. He was among the first to use aerial surveying with seaplanes to map the coast of East Antarctica, which he completed from the Weddell Sea to the Shackleton Ice Shelf, concentrating on Bouvet Island and the region from Enderby Land to Coats Land. From the seaplane brought on the 1936-1937 expedition, members took 2,200 aerial photographs, covering 6,250 km2. Mrs Christensen became the first woman to fly over the continent.
On 1 December 1927, as the leader of one of his financed expeditions, Christensen landed on and claimed the Bouvet Island (Bouvetøya) for Norway; it had previously been claimed by Great Britain, but the British soon abandoned their claim and recognized the island as Norwegian.
On the expeditions he financed between 1927 and 1937, Christensen's men discovered and surveyed substantial new land on the Dronning Maud Land and MacRobertson Land coasts. Places in Antarctica named after Christensen include the Lars Christensen Peak, the Lars Christensen Coast as well as Lars Christensen Land, also known as MacRobertson Land, where the (now closed) Russian Soyuz station operated. In addition, Ingrid Christensen Coast was named after Christensen's wife, one of the first women to visit Antarctica.
During World War II, Christensen was Counsellor of Finance at The Royal Norwegian Embassy in Washington, DC and a member of the Nortraship Council. After the War, the Thor Dahl Group, under the leadership of Christensen, regained its position as one of the leaders in the industry. The business also gained an increasing number of other shipping companies, both tankers and liner shipping.
He was decorated as a commander of the Order of Vasa. In 1917 he was appointed Commander of the Order of Dannebrog. He was appointed to knight of the Order of St. Olav in 1931 and in 1944 he received the Commander's Cross with the Star of Order of St. Olav.
Christensen was awarded a honorary doctorate at St. Olaf College.
Christensen was a fellow of the Royal Norwegian Society of Sciences and Letters and received its Gunnerus Medal, and an honorary fellow of the Norwegian Academy of Science and Letters. He was a honorary member of the Norwegian Geographic Society and in the Royal Norwegian Science Society in Trondheim and was awarded of the American Geographical Society David Livingstone Centenary Medal in 1935.