Ayres had the Colonial-style home on Kessler Boulevard, West Drive, built in 1941. The Lilly family purchase adjoining property to expand the estate, known as Twin Oaks, to . Lilly's daughter, Ruth, lived there until her death in 2009, and it remains privately owned. He also maintained a summer home, called Red Oaks, at Falmouth, Massachusetts. Lilly joined Eli Lilly and Company in November 1914 and became head of its newly formed Efficiency Division in 1916. He was responsible for the company's employee relations, payroll, and methods and standards departments. Following his enlistment in the U.S. Army during World War I, when he served in France as an officer in the medical supply service, Lilly returned to the family business in Indianapolis, where he spent the remainder of his career. In the late 1920s Lilly and his brother, Eli, established a Planning Department and developed improvements to the company's hiring procedures, employee bonus incentives, working conditions, and efficiency efforts. While Eli served as president of the company from 1932 to 1948, Joe's main interests were sales and marketing. During this period the company's growth was significant: "sales increased from $13 million to $115 million, and the number of employees grew from 1,675 to 6,912." As part of a company reorganization in 1944, Joe became executive vice president, while continuing to maintain responsibilities for the company's marketing effort, including the sales research, marketing, and distribution functions. The company's increasing size, the complexity of its business, and the resignation of Joe's son, Josiah III, in 1946, caused the company to consider a gradual leadership transition that would separate the company's ownership from its day-to-day management. Prior to his elevation to company president in 1948, Joe was president of Eli Lilly International Corporation. He remained at the helm until 1953, when Eugene Beesley succeeded him to become the first non-family member to serve as the company's president. Lilly was chairman of the board from 1953 until his death in 1966. Following his retirement as company president in 1953, Lilly devoted most of his time to various hobbies. He was known for his philanthropic activities, as well as his collections of rare books and manuscripts, gold coins, antique weaponry, stamps, works of art, and military miniatures. In 1937 Lilly, his father, J. K., and his older brother, Eli, founded the Indianapolis-based Lilly Endowment Inc. Over the years Joe donated a total of $6 million in Eli Lilly and Company stock to the endowment and Eli contributed stock valued at $2.8 million; however, their father made the largest stock contribution, a total of $86.8 million.