Labour said the Department for Transport data underlined the “devastating” impact of cuts on bus services.
Between April 2017 and March 2018, there were 10.78 million passenger journeys in Buckinghamshire, 5.4% less than in the previous year.
Bus use has declined slightly over the last eight years, with the number of passenger journeys in Buckinghamshire dropping by 2% since 2009-10, when these figures were first recorded.
Across England, there were 85 million fewer bus journeys last year, a drop of 3.2% outside London and 0.7% in the capital.
Andy McDonald, Shadow Transport Secretary, said: "These figures underline the devastating impact of Tory cuts on local bus services.
"People are being denied opportunities in work and education and are cut off from friends and family, particularly those in rural areas or from low income backgrounds.
"At the same time, cutting and withdrawing services is worsening congestion, air pollution and our impact on climate change.”
Bus fares across England have risen by 71% since 2005.
Over the same time period, bus operators’ revenues have increased by almost £1.5 billion.
The average person took 20 bus journeys in the local authority in 2017-18, down from 21 the previous year.
Brighton and Hove had the most passenger journeys per resident in the South East, while Windsor and Maidenhead had the fewest.
The number of concessionary journeys, taken by elderly and disabled passengers, increased by 3%.
Martin Tett, the Local Government Association's transport spokesman, said: “It's nearly impossible for councils to keep subsidising free travel while having to find billions of pounds worth of savings and protect other vital services like caring for the elderly and disabled, protecting children, filling potholes and collecting bins.
"Faced with significant funding pressures, many across the country are being forced into taking difficult decisions to scale back services and review subsidised routes."
A Department for Transport spokesman said: "It is for councils to decide which bus operations to support in their areas, but we help to subsidise costs through around £250 million worth of investment every year.
"£42 million of this is already devolved to local authorities and a further £1 billion funds the free bus pass scheme, benefiting older and disabled people across the country."