[10] However, MassMutual's path followed a different course when an 1851, Massachusetts state law required all insurance companies to take an initial stock subscription of $100,000.

As westward expansion grew, fueled by excitement from the California Gold Rush (1848–1855) and railroad development, MassMutual agents began to sell high-premium insurance policies to railway and steamship workers, gold-rush adventurers, and people relocating south of the Mason–Dixon line.

[citation needed] Across the industry, sales of life insurance policies began to substantially increase by the late 1850s.

[16] Another effort that contributed to the increased selling of life insurance policies was the passing of a non-forfeiture law by the Massachusetts legislature in 1861.

[23][24] The 1906 Armstrong Committee investigation in 1905 uncovered the questionable financial practices of several New York life insurance companies.

While MassMutual was not a specific target, the investigation required insurance companies to distribute dividends annually, to restrict the size of agents' commissions, and to regulate the nature of their investments.

[26] As the Armstrong Investigation settled, all MassMutual policies issued after October 1, 1907 earned value according to the American Experience Table of Mortality and 3% interest.

As the economy plunged 22% in a matter of days and 30% of the country's workforce was faced with unemployment, death claims and policy lapses drastically increased due to an unusual number of suicides and general economic hardship.

[32] As the Great Depression faded from the United States in 1939, the start of World War II spurred the country's industrial and economic progression, workforce and insurance demands.

[33] Once the United States became heavily involved with World War II and unemployment significantly decreased, MacLean oversaw the development of insurance products and services for workers.

As jobs were quickly created and unions strengthened, MassMutual entered the group marketplace in 1946, offering policies and managing pensions.

The first group product was a pension and insurance policy for Brown-Forman Distillers – the Kentucky-based company that produced Jack Daniel's whiskey.

[36] In addition, MassMutual instituted a training program for field agents and encouraged its workers to complete the American College Chartered Life Underwriter designation.

[46] In addition to universal life policies being introduced in 1981, MassMutual's group pension unit was upgraded to a division the same year.

[47] By early 1984, the US economy rebounded and the country entered one of the longest periods of sustained economic growth since World War II.

[48] The group pension division's assets had reached $5 billion – making MassMutual one of the biggest managers in the United States.

[53] MassMutual continued its diversification efforts through 1996 by creating Antares Leveraged Capital Corporation — a commercial finance unit providing lending services.

[67] Back in the United States, MassMutual Trust Company was created in 2000, offering investment services and estate planning, in Hartford, Connecticut.

[76] In November 2021, MassMutual entered into a strategic partnership with the UK-based independent power producer Low Carbon, led by CEO Roy Bedlow.

[77][78] In April 2022, Massachusetts-based Fidelity Investments announced it would be the record keeping service provider of MassMutual's corporate 401K plan.

[82] MassMutual owns Tremont Capital Management, a feeder fund that fed investors' money to Bernard Madoff's Ponzi scheme and ultimately lost $3.3 billion in the resulting scandal.

[83] Irving Picard, the receiver who is recovering money from the Ponzi scheme, extracted a settlement from Tremont worth over one billion dollars.