After his second term as governor ended in 1903, McMillin established an insurance business in Nashville.
 He remained active in Democratic Party politics, however, serving as an elector in every presidential election between 1876 and 1932, with the exception of the 1916 election, and attending every Democratic National Convention during the same period, with the exception of the 1920 convention.

In 1912, a divided Democratic Party nominated McMillin as the party's candidate for governor in hopes of unseating Republican Ben W. Hooper.
 The party was divided over the prohibition issue, with one faction, the "Independent Democrats" (who supported statewide prohibition), having withdrawn from the party and formed a coalition (the "Fusionists") with Republicans.
 McMillin represented the other faction, the "Regular Democrats," who believed the state's largest cities should be exempt from prohibition.
 He was defeated in the general election, winning just 116,610 votes to Hooper's 124,641.
 One factor in the electoral loss may well have been the death of McMillin's only son in Bristol, Tennessee, who took ill at the end of October.
 When the doctors lost hope, candidate McMillin and his wife stayed at his son's bedside for nearly a week, cancelling all engagements.

In 1913, President Woodrow Wilson appointed McMillin Envoy Extraordinary and Minister Plenipotentiary to Peru.
 Shortly after arriving in the Peruvian capital, Lima, he helped negotiate an "Advancement of Peace" that formalized relations between the two countries.

In 1919, McMillin was appointed Minister to Guatemala.
 A few months after his arrival, a revolt erupted against unpopular president Manuel Estrada Cabrera, with Cabrera eventually surrendering to McMillin to avoid capture by supporters of Carlos Herrera.
 The American embassy was damaged during Herrera's five-day bombardment of the capital.
 Herrera would also be deposed in a coup before the end of McMillin's tenure.

Upon returning to Tennessee, McMillin again sought his party's nomination for governor.
 His chief opponent was Clarksville farmer and public education advocate Austin Peay.
 Although the 77-year-old McMillin campaigned vigorously, Peay had the support of rising political boss E. H. Crump, and edged McMillin for the nomination, 63,940 votes to 59,922.

After his defeat in the 1922 campaign, McMillin returned to his insurance business.
 He died in Nashville on January 8, 1933, and is buried in the city's Mount Olivet Cemetery.
After his second term as governor ended in 1903, McMillin established an insurance business in Nashville.  He remained active in Democratic Party politics, however, serving as an elector in every presidential election between 1876 and 1932, with the exception of the 1916 election, and attending every Democratic National Convention during the same period, with the exception of the 1920 convention.
In 1912, a divided Democratic Party nominated McMillin as the party's candidate for governor in hopes of unseating Republican Ben W. Hooper.  The party was divided over the prohibition issue, with one faction, the "Independent Democrats" (who supported statewide prohibition), having withdrawn from the party and formed a coalition (the "Fusionists") with Republicans.  McMillin represented the other faction, the "Regular Democrats," who believed the state's largest cities should be exempt from prohibition.  He was defeated in the general election, winning just 116,610 votes to Hooper's 124,641.  One factor in the electoral loss may well have been the death of McMillin's only son in Bristol, Tennessee, who took ill at the end of October.  When the doctors lost hope, candidate McMillin and his wife stayed at his son's bedside for nearly a week, cancelling all engagements.
In 1913, President Woodrow Wilson appointed McMillin Envoy Extraordinary and Minister Plenipotentiary to Peru.  Shortly after arriving in the Peruvian capital, Lima, he helped negotiate an "Advancement of Peace" that formalized relations between the two countries.
In 1919, McMillin was appointed Minister to Guatemala.  A few months after his arrival, a revolt erupted against unpopular president Manuel Estrada Cabrera, with Cabrera eventually surrendering to McMillin to avoid capture by supporters of Carlos Herrera.  The American embassy was damaged during Herrera's five-day bombardment of the capital.  Herrera would also be deposed in a coup before the end of McMillin's tenure.
Upon returning to Tennessee, McMillin again sought his party's nomination for governor.  His chief opponent was Clarksville farmer and public education advocate Austin Peay.  Although the 77-year-old McMillin campaigned vigorously, Peay had the support of rising political boss E. H. Crump, and edged McMillin for the nomination, 63,940 votes to 59,922.
After his defeat in the 1922 campaign, McMillin returned to his insurance business.  He died in Nashville on January 8, 1933, and is buried in the city's Mount Olivet Cemetery.
Benton McMillin