The Pensions Regulator, which is responsible for supporting firms as they automatically place employees into workplace pensions, confirmed the milestone figure has been reached.
The shake-up started last autumn with larger companies, amid fears that people are living for longer but failing to put enough cash aside for their later years.
An estimated 11 million people are not saving enough to achieve the level of income they are likely to want for a comfortable retirement and official figures released earlier this week showed that private pension saving slid to record lows just before the reforms were launched.
Minister for Pensions Steve Webb said: "Today is a real landmark in this quiet revolution, finally reversing decades of decline in pension saving. This is the biggest change to the pensions system for a century. The money workers save is being matched by contributions from their employers and topped-up by tax relief, helping them put money aside for their retirement, many for the first time."
The biggest firms started placing workers into schemes from October. The staging process is intended to give smaller companies who may have less experience of pension schemes more time to adjust.
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Builders merchants Travis Perkins is among employers who have helped get the number of pension savers to the one million mark. The company has placed around 9,000 of its workers into a pension scheme.
Geoff Cooper, chief executive of the Travis Perkins Group said: "Encouraging every one of our 24,000 employees to save for the future is very important for us, and I am proud that our company has played a part in reaching this important first million."
Pensions experts have said that the country would be on course for a crisis in paying for its old age if auto-enrolment had not been introduced. Just 35% of men and 32% of women aged between 16 and 64 were actively saving onto a private pension in 2011/12, marking a new low point for records held by the Office for National Statistics (ONS).