The Parker "21" was a school pen that followed the design of the Parker 51, but made in cheaper plastic and with an alloy 8-metal nib rather than gold. There are two distinctive designs and five different clips in the 21-line. The first design, Mark I (1948–1951), had a larger section hole, showing more of the feed compared to the Parker 51. Also, the nib was not as tubular as the Parker 51 nib. The nib and feed were fitted tightly into the filler unit with the section screwing loosely over the top — the screw had a rather more "hollow" space in it and, because of this, could contain and hold more ink. Mark I had the body imprint "Parker 21 Made in USA" and also a date imprint. Twist action pencils were also offered. In 1951, the Parker "21" Special was introduced with a stainless steel cap.

The trouble with this first design was that the ink dried on the feed too quickly if the pen was left unused for a while, but also that it sometimes too willingly released ink and created a nice blob; it also frequently leaked in the cap. So, in 1952, the complete Parker 21 line was redesigned. It is important, though, to remember that all parts between the two Parker 21 designs are not interchangeable. The nib and feed can be exchanged, the section and filler cannot. The Mark II's redesigned section fitted snugly over the nib and feed — they are easily told apart since the Mark I nib and feed were attached to the filler unit when the section was unscrewed; Mark II "stuck" in the section.

By 1953 the Parker 21 Super was introduced. It sported a clip that was to survive well into the 1990s on another pen, introduced in 1960, the Parker "45". This new clip was more stylised than the previous and had fewer "feathers". The first Parker 21 Super pens had, in fact, the Ridge clip but with the addition of the Parker 45-clip and an imprint. But soon, the new clip was adopted to the line. Also in 1953, a Parker "21" desk pen was offered in black.

While the Parker 45 was introduced in 1960 at a price of $5, it was in fact cheaper than the Parker 21 and sported a gold nib, as opposed to the octanium nib of the Parker 21. The new cartridge filling system also won the public's approval and, in 1965, the Parker 21 was discontinued.