Born on 29 October 1909 in Forest Hill, London, the youngest of four children, Daniell developed an interest in motorcycles as a teenager after riding his father's AJS big-twin. His first competitive event was a hill climb entered on impulse, which he won outright, setting the course of his career. His early machinery was a 1925 Model 18 Norton; racing it on the developing Crystal Palace circuit, he won the 1929 Crystal Palace Championship and set the lap record. During this period he was studying motor engineering in Chelsea and was already tuning and preparing his own machines.
His Isle of Man ambitions were crystallised when he entered the Manx Grand Prix for the first time in 1930. His first two years on the Island ended in retirement, but in 1932 he finished second in the Senior Manx Grand Prix and in 1933 he won the Senior race at an average speed of 76.98 mph — his first major victory.
Daniell first appeared at the Isle of Man TT itself in 1934, riding for AJS. His two seasons with AJS machinery proved relatively unsuccessful, but in 1937 — back on Nortons, now tuned by his future brother-in-law Steve Lancefield — he finished fifth in both the Junior and Senior TT events, attracting attention from the official Norton works team.
Invited into the Norton works squad for 1938, Daniell delivered the most celebrated performance of his career at the Senior TT. Racing Stanley Woods on a Velocette in a close and sustained duel, Daniell equalled the lap record on the fifth circuit and then shattered it on the final lap, recording a time of 24 minutes 52.6 seconds — a lap at 91.00 mph. It was the first sub-25-minute lap in the history of the course and the first time the Mountain circuit had been lapped at over 91 mph. He won the race by 14.4 seconds. This record stood for twelve years.
After a disrupted 1939 season and the wartime break, Daniell returned to the Island in 1947. Despite strong competition from younger riders — Artie Bell, his Norton team mate, finished just 22 seconds behind — Daniell won the Senior TT for a second time. The 1948 TT brought retirement in both the Junior and Senior races, with handling and engine problems blunting his challenge. In 1949, however, he came back emphatically: he lay as low as fifth in the early laps of the Senior race before moving through the field, ultimately winning when Les Graham's AJS Porcupine suffered a magneto failure at Hillberry with two miles remaining. It was his third and final TT victory.
His 1950 TT appearances — his last — saw him finish fifth in the Senior and third in the Junior. Geoff Duke's arrival in the Norton team had shifted the balance; Duke won the Senior and broke Daniell's twelve-year lap record at 93.33 mph that year.
In the inaugural FIM Motorcycle World Championship of 1949 Daniell competed in both the 350 cc and 500 cc classes on Norton machinery. His Senior TT victory was the opening round of the new 500 cc championship. He finished sixth overall in the 500 cc standings with 17 points, while Leslie Graham took the title. In the 350 cc class he finished 12th. In 1950 he continued in both classes, finishing 12th in the 500 cc standings and 8th in the 350 cc. His 1949 Senior TT win is recognised as the first race win of the modern 500 cc — subsequently MotoGP — class.
Daniell is credited with coining one of motorcycling's most enduring technical terms. When he first rode the new Norton duplex loop frame designed with input from Rex McCandless, he remarked during the prize-giving at the Villa Marina following the 1949 TT that it was like "riding on a featherbed" compared to the old Norton rigid frame, which he called the "garden gate." The name Featherbed frame has been used to describe the design ever since.
Contemporary accounts describe Daniell as a quiet, methodical competitor who prepared his machines carefully and took his physical conditioning seriously, working out at a gymnasium particularly after the Christmas break. He was also noted as a skilled storyteller and public speaker.
After retiring from motorcycle racing around 1951 Daniell competed in Formula 3 events driving an Emeryson single-seater, from approximately 1951 to 1953. He then returned full-time to the Norton dealership he ran in Forest Hill, London. He served on the board of the British Motorcycle Racing Club (BEMSEE) from 1949 and was Chairman of its board of directors at the time of his death — a tenure that was unlikely to be exceeded. Harold Daniell died on 19 January 1967, aged 57.