A native of Summit, New Jersey, Lummis studied theatre arts in Los Angeles at the Martha Oatman School. His first professional engagement, at the age of twenty-four, was with the Russell Stock Company in Redlands, California. He remained a regional actor until his Broadway debut in 1943.
Lummis was cast in his first screen role, a minor appearance, at the age of forty-two in the Burt Lancaster and Barbara Stanwyck film Sorry, Wrong Number. After a few other motion picture appearances, some uncredited, Lummis was cast as a police superintendent in the television series Racket Squad in the 1952 episode "The Strange Case of James Doyle" Hugh Beaumont, later the father, Ward Cleaver, on the sitcom Leave It to Beaver, narrated a third of the episodes of this series, which starred Reed Hadley as Captain John Braddock. That same year, Lummis appeared as Paul Clarkson in the episode "Where There's a Will" in the detective series, Mr. and Mrs. North, starring Richard Denning. In 1954, he appeared as police Sergeant Jack Gotch in "The Big Trunk" episode of Jack Webb's Dragnet. In 1958, he appeared as Jonas Warman in the episode "The Healer" of NBC's M Squad crime drama, starring Lee Marvin.
Lummis appeared in numerous anthology series, including CBS's Four Star Playhouse anthology series as a prison warden in "Vote of Confidence" and as Whit Lonigan in "A Championship Affair" (both in 1954). In 1955, he appeared twice on CBS's Schlitz Playhouse of Stars as a newspaper editor named Cartwright in "The Last Pilot Schooner" and as Arthur Healy in "Ambitious Cop". Between 1954 and 1956 Lummis was cast in four separate roles, two as a physician, on NBC's The Loretta Young Show. He appeared as the character Nigel in the 1956 episode "Temptation" on CBS's Lux Video Theatre In 1955, he portrayed an executive officer in the episode "Sky Pilot" of the CBS military anthology series, Navy Log. From 1953-1957, he was cast four times on the CBS anthology, General Electric Theater, in the episodes "Best Seller", "My Wife, Poor Wretch", "Too Good with a Gun", and "I Will Not Die". He was cast as Colonel Duncan Smuthe in "The Tichborne Claimant" of the NBC series the Joseph Cotten Show, also known as 'On Trial. In 1958, Lummis appeared on NBC's Shirley Temple's Storybook in the episode "The Nightingale". From 1956-1958, he appeared three times on CBS's Alfred Hitchcock Presents anthology, as Tom Ackley in "Crack of Doom", Charles Blanchard in "Mr. Blanchard's Secret", and as Police Sergeant Oliver in "Listen, Listen!" In 1958, he appeared twice – as Dr. Engle in "Before I Die" and as Colonel Brecker in "Bitter Heritage" – on CBS's Playhouse 90. In 1958 and 1959, he appeared as a prosecutor in "The Lady Takes the Stand" and as Lord Meredith in "A London Affair" of NBC's Goodyear Theatre
Lummis was assigned his first western role as banker Jonathan Wilkins in the 1953 episode entitled "Trouble in Town" of The Lone Ranger series. Other western roles followed in 1956 as Stephen Austin in The First Texan and 1957 as General Rogers on the syndicated series The Adventures of Jim Bowie, loosely based on the life of the Alamo defender Jim Bowie. Also in 1957, he appeared in the episode "The Fugitive" of another syndicated western, Man Without a Gun, starring Rex Reason and Mort Mills. In 1958, Lummis was cast in an uncredited role as a padre in Audie Murphy's film, From Hell to Texas. That same year, he appeared as Jabez Lord in the episode "Hunter's Moon" of the NBC series Buckskin starring child actor Tommy Nolan. The nest year, he guest starred in "Excitement at Milltown" of Rod Cameron's syndicated State Trooper. He was cast as a Judge Randall in the 1959 episode "Gone But Not Forgotten" of the CBS series Yancy Derringer, starring Jock Mahoney. In 1960, he appeared as Gideon Templeton in the episode "Path of the Eagle" of the NBC western series, Riverboat, starring Darren McGavin.
His Law of the Plainsman episodes include the following :"Prairie Incident" (1 October 1959)
"A Matter of Life and Death" (15 October 1959)
"The Hostiles" (22 October 1959)
"Blood Trails" (5 November 1959)
"Appointment in Santa Fe" (19 November 1959)
"The Gibbet" (26 November 1959)
"The Innocents" (10 December 1959)
"Endurance" (14 January 1960)
"Dangerous Barriers" (10 March 1960)
In 1960, Lummis was cast twice on the syndicated western series, Death Valley Days, first as Lew Wallace, governor of the New Mexico Territory and the author of the novel, Ben Hur: A Tale of the Christ, in the episode "Shadows on the Window". He then portrayed the character John De La Mar in "City of Widows."
In 1960, he appeared twice on Chuck Connors's ABC western The Rifleman as Colonel Craig in "The Lariat" and as Jake Shaw in "The Illustrator". That same year, Lummis appeared twice on the syndicated Death Valley Days, as (1) the historical figure, Lew Wallace in "Shadows on the Window", with Martin Braddock as Billy the Kid, and (2) as De La Mar in "City of Widows". In 1961, he played a judge in "Killer Without Cause" of the NBC series, Laramie. He appeared that same year in two Warner Brothers westerns on ABC: as Silas Rigsby in "Trouble at Sand Springs" of Will Hutchins's Sugarfoot and as Frank Collins, father of a wayward youth played by Richard Evans, in the episode "The Young Fugitives" of Clint Walker's Cheyenne His next western role was as Dr. Burroughs in the 1962 episode "The Ross Bennett Story" of NBC's The Wide Country, starring Earl Holliman and Andrew Prine.
In 1963, he was cast as Horatio Turner in "The Money Cage" of NBC's 90-minute western The Virginian. Lummis starred as Jasom Simms in "Green, Green Hills" (1962) and Thomas Fenton Giler (uncredited) in "Down There, the World" (1963) of NBC's modern western series, Empire, starring Richard Egan. In 1963, he was cast as Clayton Emory in episode "The Chooser of the Slain" of the short-lived Warner Brothers western series The Dakotas, starring Larry Ward and Chad Everett. Still another 1963 role was as Colonel Bob Grainger in "Fracas at Kiowa Flats" of the NBC series, Temple Houston, starring Jeffrey Hunter.
Lummis appeared three times in the NBC and ABC western, Wagon Train: as Major Barham in "the Martha Barham Story" (1959) with Ann Blyth in the title role, as T.J. Gingle in "The John Turnbull Story" (1962), with Henry Silva, and as the Reverend Philip Marshall in "The Myra Marshall Story" (1963), starring Suzanne Pleshette. He appeared four times on NBC's Bonanza: as Colonel Metcalfe in "Escape to Ponderosa" (1960), as attorney Hiram Wood in "the Secret" (1961), as Colonel Abel Chapin in "The Legacy" (1963), and as Judge O'Hara in "The Dilemma" (1965). The 1965 Bonanza appearance was Lummis' last western role for a full decade, when he appeared on February 3, 1975, as 71-year-old Mr. Holmby in the episode ""The Angry Land", one of the last episodes of CBS's longest-running western, Gunsmoke, starring James Arness. "the Angry Land" was the penultimate Gunsmoke appearance for Arness as well as Lummis's final screen role.
Lummis appeared in other television series and films during his career, including:Tangier Incident (1953)
Julius Caesar (1953) as Messala
I Love Lucy as Bill Parker in "Lucy Has Her Eyes Examined" (1953), as Mel Eaton in "Lucy Writes a Novel" (1954), and as Mr. Sherman in "Hollywood at Last" (1955)
Lassie as Dr. Walter Stewart in three episodes, "Gramps" (1954) and "The Injury" and "The Snake" (CBS, both 1955)
The Glenn Miller Story as Colonel Spaulding (uncredited), 1954
Demetrius and the Gladiators as Magistrate, 1954
Crusader as Ray Talbot in "Nine Priceless Objects" (1956)
You Are There, Walter Cronkite's "The Bank Holiday Crisis of March 6, 1933" (CBS, 1957)
Markham as Howard Fulton in "The Father" (CBS, 1959)
Angel as Mr. Mathews in "Angel's Temper" and "The Museum" (CBS, both 1960)
The Lawless Years as a judge in "The Prantera Story" (NBC, 1960)
Spartacus as Symmachus, uncredited (1960)
Elmer Gantry as the newspaperman Eddington (1960)
Adventures in Paradise as Charles Fouchet in "Please Believe Me" (ABC, 1962)
Thriller as Clarence in "The Cheaters" (1960) and as Millard Braystone in "Cousin Tundifer" (NBC, 1962)
77 Sunset Strip as Guy Winters in "Shadow on Your Shoulder" (ABC, 1962)
Ripcord as Dr. Chapman in "Flight for Life" (Syndicated, 1962)
Walt Disney's Wonderful World of Color as a commissioner in the two-part "The Mooncussers" (NBC, 1962)
Arrest and Trial as Dr. Murray in "Funny Man with a Monkey" (ABC, 1964)
Voyage to the Bottom of the Sea as Gustave Reinhardt in "The Last Battle" (ABC, 1965)
The Time Tunnel' as Gladstone in "Night of the Long Knives" (ABC, 1966)
This Is the Life as Judge Brandt in "Parents, Too, Can Be Delinquents" (Syndicated, 1967)
Moonfire as Fuentes in film about truckers battling a Nazi hiding out in Mexico, with Richard Egan, Richard Bull, Charles Napier, and boxer Sonny Liston (1973)
Lummis maintained a ranch in Malibu, CA during his acting career. He was married to Dorothy L. Lummis (April 11, 1912—January 21, 1997), a Pennsylvania native who resided in Bryn Mawr in Delaware County at the time of her death. Lummis himself died at the age of eighty-four in Santa Monica in Los Angeles County, California. His son, Dayton Lummis, Jr. (born ca. 1936), of Santa Fe, New Mexico, is a former museum curator and author of numerous nonfiction works on the American West.