Putnam graduated from Swarthmore College in 1963, won a Fulbright Fellowship to study at Balliol College, Oxford, and went on to earn master's and doctorate degrees from Yale University, the latter in 1970. He taught at the University of Michigan until going to Harvard in 1979, where he has held a variety of positions, including Dean of the Kennedy School, and is currently the Malkin Professor of Public Policy.

His first work in the area of social capital was Civic Traditions in Modern Italy, a comparative study of regional governments in Italy which drew great scholarly attention for its argument that the success of democracies depends in large part on the horizontal bonds that make up social capital.

In 1995 he published '"America's Declining Social Capital" in the Journal of Democracy. The article was widely read and garnered much attention for Putnam, including an invitation to meet with then-President Bill Clinton. Some critics argued that Putnam was ignoring new organizations and forms of social capital; others argued that many of the included organizations were responsible for the suppression of civil rights movements and the reinforcement of anti-egalitarian social norms. Over the last decade and a half, the United States had seen an increase in bowlers but a decrease in bowling leagues.

In 2000, he published Bowling Alone: The Collapse and Revival of American Community, a book-length expansion of the original argument, adding new evidence and answering many of his critics. Though he measured the decline of social capital with data of many varieties, his most striking point was that many traditional civic, social and fraternal organizations — typified by bowling leagues — had undergone a massive decline in membership while the number of people bowling had increased dramatically.

Putnam makes a distinction between two kinds of social capital: bonding capital and bridging capital. Bonding occurs when you are socializing with people who are like you: same age, same race, same religion, and so on. But in order to create peaceful societies in a diverse multi-ethnic country, one needs to have a second kind of social capital: bridging. Bridging is what you do when you make friends with people who are not like you, like supporters of another football team. Putnam argues that those two kinds of social capital, bonding and bridging, do strengthen each other. Consequently, with the decline of the bonding capital mentioned above inevitably comes the decline of the bridging capital leading to greater ethnic tensions.

Critics such as sociologist Claude Fischer argue that (a) Putnam concentrates on organizational forms of social capital, and pays much less attention to networks of interpersonal social capital; (b) neglects the emergence of new forms of supportive organizations on and off the Internet; and (c) the 1960s are a misleading baseline because the era had an unusually high number of traditional organizations.

Since the publication of Bowling Alone, Putnam has worked on efforts to revive American social capital, notably through the Saguaro Seminar, a series of meetings among academics, civil society leaders, commentators, and politicians to discuss strategies to re-connect Americans with their communities. These resulted in the publication of the book and website, Better Together, which provides case studies of vibrant and new forms of social capital building in the United States

In recent years, Putnam has been engaged in a comprehensive study of the relationship between trust within communities and their ethnic diversity. His conclusion based on over 40 cases and 30 000 people within the United States is that, other things being equal, more diversity in a community has a correlation [expressed as a beta equal to 0.04 in a multiple regression analysis (see Putnam, 2007), to less trust both between and within ethnic groups. Although only a single study and limited to American data, it claims to put into question both contract theory and conflict theory in inter-ethnic relations. According to conflict theory, distrust between the ethnic groups will rise with diversity, but not within a group. According to contact theory, distrust will decline as members of different ethnic groups get to know and interact with each other. Putnam describes people of all races, sex and ages as "hunkering down" and going into their shells like a turtle. For example, he did not find any significant difference between 90 year olds and 30 year olds.

Low trust with high diversity not only affects ethnic groups, but is also associated with:

• Lower confidence in local government, local leaders and the local news media.
• Lower political efficacy — that is, confidence in one's own influence.
• Lower frequency of registering to vote, but more interest and knowledge about politics and more participation in protest marches and social reform groups.
• Less expectation that others will cooperate to solve dilemmas of collective action (e.g., voluntary conservation to ease a water or energy shortage).
• Less likelihood of working on a community project.
• Less likelihood of giving to charity or volunteering.
• Fewer close friends and confidants.
• Less happiness and lower perceived quality of life.
• More time spent watching television and more agreement that "television is my most important form of entertainment".

Putnam published his data set from this study in 2001 [1] [2] and subsequently published the full paper in 2007.

Putnam has been criticized for the lag between his initial study and his publication of his article. In 2006, Putnam was quoted in the Financial Times as saying he had delayed publishing the article until he could "develop proposals to compensate for the negative effects of diversity" (quote from John Lloyd of Financial Times) [3]. In 2007, writing in City Journal, John Leo questioned whether this suppression of publication was ethical behavior for a scholar, noting that "Academics aren’t supposed to withhold negative data until they can suggest antidotes to their findings." [4] On the other hand, Putnam did release the data in 2001 and publicized this fact [5]. The proposals that the paper contains are located in a section called "Becoming Comfortable with Diversity" at the end of his article. This section has been criticized for lacking the rigor of the preceding sections. According to Ilana Mercer "Putnam concludes the gloomy facts with a stern pep talk". [6]

Robert Putnam has been elected to the American Academy of Arts and Sciences (1980), the Council on Foreign Relations (1981), the National Academy of Sciences (2001), and the American Philosophical Society (2005). He was the President of the American Political Science Association (2001—2002). He has received honorary degrees from Stockholm University, Ohio State University, University of Antwerp, University of Edinburgh, among others. He is the recipient of the Wilbur Cross Medal of Yale Graduate School of Arts and Sciences for outstanding career achievement (2003). In 2006 Robert Putnam received the Johan Skytte Prize for the most valuable contribution to political science.