The ship was constructed in the French Société Nouvelle des Forges et Chantiers de la Méditerranée shipyard in Toulon; her hull was laid in 1866 and launched in January 1867.

During the Franco-Prussian War in 1870–1871, the ship formed part of the main German squadron commanded by Vizeadmiral (Vice Admiral) Eduard von Jachmann.

Engine trouble, however, plagued the ship and two of the other three vessels in the squadron; as a result, they made only two sorties from the port of Wilhelmshaven to challenge the French blockade.

Following the acquisition of the small ironclad warships Arminius and Prinz Adalbert, which were only usable in coastal areas, the Prussian Navy sought to acquire armored vessels capable of operations on the high seas.

Ironclads were at that time a recent development and the only option for sea-going warships was the armored frigate, modeled on traditional sailing ships with a battery on the broadside.

[3] Friedrich Carl was an excellent sea boat; the ship was responsive to commands from the helm and had a moderate turning radius.

Friedrich Carl carried a number of smaller boats, including a large tender, two launches, a pinnace, two cutters, two yawls, and one dinghy.

The ship carried 624 t (614 long tons) of coal, which enabled a maximum range of 2,210 nautical miles (4,090 km; 2,540 mi) at a cruising speed of 10 knots (19 km/h; 12 mph).

A barque rig with a surface area of 2,010 square meters supplemented the steam engine, though in service they added little to the ship's performance.

The battery's roof was protected by 9-millimeter-thick (0.35 in) iron plating, intended to deflect shots that passed over the side of the ship or fragments from explosions.

She was laid down at the Société Nouvelle des Forges et Chantiers de la Méditerranée shipyard in Toulon the following year.

[8] The four ships, under the command of Vizeadmiral (Vice Admiral) Eduard von Jachmann, made an offensive sortie in early August 1870 out to the Dogger Bank, though they encountered no French warships.

Friedrich Carl and the other two broadside ironclads thereafter suffered from chronic engine trouble, which left Arminius alone to conduct operations.

[11] Friedrich Carl, Kronprinz, and König Wilhelm stood off the island of Wangerooge for the majority of the conflict, while Arminius was stationed in the mouth of the Elbe river.

Beginning in September 1872, Friedrich Carl embarked on a world cruise with the screw frigate Elisabeth and the gunboat Albatross.

Friedrich Carl, under the command of Vizeadmiral Reinhold Werner, steamed to Spanish waters along with a pair of unarmored vessels.

[15] While steaming off Alicante, Friedrich Carl encountered the rebel armed steamer Vigilante, seized the vessel, and returned it to the national government.

The German ships were joined by French, Russian, Italian, and Austro-Hungarian warships in an international demonstration condemning the murder.

The squadron went to the Mediterranean again in 1877, though this time it included Friedrich Carl, Kaiser, Deutschland, and the turret ship Preussen, along with the aviso Falke.

Friedrich Carl remained laid up for the 1878 training cruise that saw the loss of the brand-new ironclad Grosser Kurfürst in an accidental collision with Kaiser.

She returned to service in May 1879, serving as the flagship of the training squadron, which also included Kronprinz, Preussen, and Friedrich der Grosse.

The ship remained in service through 1883; later that year, the training cycle concluded with a large-scale simulated attack on Kiel, with Friedrich Carl and the other ironclads acting as an "eastern" opponent.

Neptun was formally stricken from the naval register on 22 June 1905 and sold to a Dutch ship-breaking firm in March 1906 for 284,000 gold marks.