Postum quickly became popular, making Post wealthy. The aggressive advertising, with the slogan "There's a Reason," warned against the alleged dangers of coffee and caffeine, and promoted the benefits of Postum. When imitations appeared, the company introduced a cheaper drink called Monk's Brew that was identical to Postum, but discontinued it after competitors left the market. Instant Postum appeared in 1911.^{[2]: 93–98}

Although the Post Cereal Company explicitly stated in its advertising that Postum did not taste like coffee and was not a coffee substitute,^[4] the drink enjoyed an enormous rise in sales and popularity in the United States during World War II when coffee was rationed and people sought a replacement.^[2]: 223

Postum was sometimes marketed by a cartoon ghost named Mister Coffee Nerves,^[5] who would appear in situations wherein people were shown in uncomfortable life situations (e.g., irritability, lack of sleep, lack of athletic prowess) due to their use of coffee and its negative effects. These cartoons always ended with the afflicted people switching to Postum and Mister Coffee Nerves fleeing until the next cartoon.^[6] The company sponsored the radio shows Lum and Abner, Beulah and The Aldrich Family, and the radio version of Father Knows Best.