NASA uses the metric system

Did NASA use metric units for the Mercury missions?

The Mercury project used imperial units of measurement. For example, the main dashboard on the Mercury spacecraft showed altitude in (feet):

The main dashboard of the Mercury spacecraft from the Mercury Indoctrination project, May 1959 (Source: NASA. Click full size)

They used statutory miles as a measure of distance in schematics, diagrams and other vectors in the documentation, for example these results from a conference on the results of the USA's first manned suborbital space flight on June 6, 1961:

First manned suborbital space flight in the USA - pictorial representation of the entire flight (source: public archive (in PDF format))

And even in detail to provide your employees traveling abroad (in this case Mexico) unit conversion tables from the imperial to the metric system of units in the NASA Introduction to Project Mercury and in the site manual:

Page excerpt from the introduction to the Mercury project and on-site manual, information on staff trips to Mexico (metric system of units)

The Imperial units of measure persisted throughout the Mercury program. This is the mission timeline for Mercury Redstone 4, which launched on July 21, 1961:

The units are also converted to a metric system for convenience. The original minutes of the mission communication can be found here (in PDF format).