In her first Senshi uniform, Sailor Mercury demonstrates her power over water withShine Aqua Illusion.
In the manga, Ami can dowse without any aids. Otherwise, she is not shown using any special powers in her civilian form, and may not be able to. She must first transform into a Sailor Senshi by raising a special device (pen, bracelet, wand, or crystal) into the air and shouting a special phrase, originally "Mercury Power, Make-up!" As she becomes more powerful and obtains new transformation devices, this phrase changes to evoke Mercury Star, Planet, or Crystal Power. In the anime, Sailor Mercury's transformation sequence evolves slightly over time, whether to update the background images or to accommodate changes to her uniform or a new transformation device, but all of them involve a stream of water which she whirls around her body as she spins, forming her outfit with a ripple effect.
Sailor Mercury has the power to manipulate water. For the entire first story arc, she uses her powers only to create dense clouds of mist, chilling and blinding the enemy while her allies prepare more direct attacks. In the manga she usually does this without speaking, while in the anime it is given the name Shabon Spray. In the updated renewal manga, this power is renamed Mercury Aqua Mist to match the live-action series, in which she has an offensive beam attack by that name, capable of destroying weaker enemies. She uses a total of five attacks in this series, most of which are variations on the first.
Sailor Mercury's first major offensive attack is Shine Aqua Illusion, introduced in the second story arc, which can be used as a simple projectile, to freeze the enemy in solid ice, or to create a defensive barrier. Aside from variations on her other powers (mostly improving their strength with the addition of "Freezing" or "Snow"), her next named attack is Mercury Aqua Mirage, used during the third arc of the manga and again in the special side-story "Ami's First Love" (manga and anime). Her final and greatest power comes in the fourth story arc, when she takes on her second Senshi form (Super Sailor Mercury in the anime). At this stage she acquires a special weapon, the Mercury Harp, and with it Mercury Aqua Rhapsody, which is her primary attack for the duration of the story. In the anime Mercury gains the harp deep within her subconscious. The manga took a different approach with the Mercury Harp making it more of an item rather than a harp formed due to the attack. Furthermore Sailor Mercury gained the harp through her power guardian. In the manga the Mercury Harp, as well as the other inner senshi's weapons have their own "wills" and can even speak and give input.
In addition to her own powers, Sailor Mercury has more non-magical items than any other Senshi. Early on in the series she makes frequent use of an extremely powerful "Micro-miniature Super Computer" that enables her to make special calculations, scan her surroundings, track the movements of allies as well as foes, and determine her enemies' weak points. The computer works in sync with her Mercury Goggles, which analyze the area around her and display information in front of her eyes and on the Computer. The visor may be a hologram of some sort; it materializes across her face when she touches her earring. The manga sometimes shows her wearing a small microphone connected to her earring, which she uses to communicate with Luna at the Senshi hideout during the manga's first arc. All of these devices gradually fall out of use as the series progresses. In the live-action series, like most of the other Senshi, she receives a tambourine-like weapon called the Sailor Star Tambo. She is one of two Senshi to use it for a named attack (Mercury Aqua Storm), and later can transform it into a sword. She also creates swords out of water, both as Sailor Mercury and as Dark Mercury.
In the manga, the Mercury Crystal and Mercury Harp are among her most significant magical possessions. The former is her Sailor Crystal and the source of all of her power, which becomes especially important in the fifth story arc. The harp's form is actually that of a lyre, a stringed instrument smaller than a harp, which according to Roman myth was invented by the god Mercury.