Iron Mountain Incorporated

The full register, from 13F filings

The Register tab above is hand research: a dozen holders, each traced to the filing it came from. This asks the same question of the whole market for the quarter ended 31-mar-2026, where 1,071 managers reported a position. Broad where the research is deep, and shallow in the ways a 13F always is.

Built from 13F filings, which report US listed, long, US custodied positions only. A register from this source is a floor rather than a level: a holder's true position can be far larger, as BlackRock's ASML stake shows at 1.28m shares in a 13F against 26.3m in its 13G/A. Holdings held through derivatives, outside US custody, or below a manager's reporting obligation do not appear. Where a holder is marked as a family, its total sums several separate filing entities and describes something no single filing describes; the constituent entities and their share counts are listed so the total can be checked. That roll-up can legitimately exceed the family's own Schedule 13G, because a 13G is filed by one legal entity: Vanguard reports Alphabet through ten advisory entities, and The Vanguard Group Inc itself reported zero after its January 2026 disaggregation.

What a reader needs to know to read these numbers

• Iron Mountain is NOT an UPREIT. Its 10-Q (30 Jun 2026) and 10-K (FY2025) contain zero hits for 'operating partnership', 'UPREIT', 'OP unit', or any language describing an umbrella partnership or units exchangeable into common stock. Iron Mountain holds and operates its real estate directly (and through consolidated and equity-method subsidiaries), not through an operating partnership with outside limited partners. company.shares_outstanding is therefore simply the listed common shares outstanding from the 10-Q cover page (297,702,812 as of 24 Jul 2026), with no OP-unit adjustment needed and no holder in this register carrying units instead of shares. This is unlike Simon Property Group, whose proxy reported a large share of its 'shares' figure as exchangeable operating-partnership units.
• Holdings side: exactly one item, the Frankfurt data center joint venture with AGC Equity Partners (20% equity-method interest, unconsolidated, carrying value $78,209,000 as of 30 Jun 2026). This was confirmed by reading the investments note in both the fresh 10-Q and the FY2025 10-K, which each state it is the ONLY joint venture accounted for by the equity method. Direction: this runs from Iron Mountain OUTWARD into a venture it does not control, so it is a genuine holding.
• Two other named ventures were searched and explicitly excluded, with the direction stated for each. (1) Web Werks (India data center operator): its constituent entities (Web Werks India Private Limited, Web Werks Infra2 Private Limited, Web Werks Fiber Private Limited, Web Werks Infrastructure Private Limited, Web Werks Internet Services Private Limited, One Web Werks Datacenter Private Limited) all appear in Iron Mountain's Exhibit 21.1 list of consolidated subsidiaries, and the current equity-method investments note does not list a Web Werks JV. Consolidated subsidiaries are wholly owned or majority-controlled operating entities, not a cross-holding stake, so Web Werks is excluded here as a wholly/majority-owned subsidiary rather than an equity investment (see unknowns for the residual uncertainty on any minority partner). (2) The Iron Mountain Data Centers Virginia 9 JV, LP (formed Q2 2026, NCI of approximately $49,900,000 on the 30 Jun 2026 balance sheet) and the Iron Mountain Data Centers Arizona 3 JV, LP (formed Dec 2025, initial NCI of approximately $74,800,000 per the FY2025 10-K): in both cases the 10-Q/10-K language is 'we entered into an agreement with a partner to form OUR [JV]... noncontrolling interest in OUR Condensed Consolidated Balance Sheet.' This is the Aon-type reverse case explicitly flagged in the brief: Iron Mountain consolidates these ventures and a third-party PARTNER holds the real minority stake in an Iron Mountain-controlled entity. That is a holding the partner has in Iron Mountain's own group, not a holding Iron Mountain has in someone else, so neither is added to the holdings array.
• Cohen & Steers, Inc. (CIK 0001284812, the parent) files its own 13F-HR directly for Iron Mountain (no 13F-NT/successor-entity resolution needed here, unlike the Digital Realty case cited in the brief). Its Q2 2026 filing DOES report the <value> field in thousands of dollars (raw $1,518,214 implying $0.13/share against a real ~$126.3/share), confirmed and corrected to $1,518,214,000 in this file.
• T. Rowe Price Associates, Inc. (CIK 0000080255) also files its <value> field in thousands (raw $80,159 implying $0.13/share), confirmed and corrected to $80,159,000. T. Rowe Price Investment Management, Inc. (CIK 0001897612) was checked separately and holds no IRM position this quarter.
• Implied per-share price across every filer checked (Vanguard family, BlackRock, Geode, FMR, Morgan Stanley, Goldman Sachs, Capital Group, Cohen & Steers, T. Rowe Price) clusters tightly around $126.3, which is treated here as Iron Mountain's real per-share price as of 30 Jun 2026 for the purpose of validating the thousands-scale checks. State Street ($127.14) and JPMorgan ($130.0) are slightly higher but the same order of magnitude, so neither was treated as a scale error.
• The proxy's own 5%-holder table (DEF 14A filed 24 Mar 2026, 'as of March 4, 2026') cites Schedule 13G/A filings for Vanguard (13.9.6% owned per that stale 13 Feb 2024 filing, 46,275,110 sh), BlackRock (10.2%, 29,851,678 sh, per a 10 Sep 2024 13G/A), Cohen & Steers (6.5%, 19,106,540 sh, per a 13 Feb 2026 13G/A) and State Street (5.7%, 16,568,963 sh, per a 17 Oct 2024 13G). Per the brief's instruction to prefer fresh 13F over stale 13G, this file uses the fresh Q2 2026 13F-HR figures for all four instead (each cited above in method_note), keeping the proxy's stale figures on record here for comparison.
• No founding-family holder was found. Iron Mountain's board includes one unusually large individual director stake (Kent P. Dauten, 1,207,681 shares, ~0.41% of the company), but he is a long-tenured director (since 1997, joined via the 1997 HIMSCORP acquisition) and Chairman of an unrelated private investment firm (Keystone Capital), not a member of an Iron Mountain founding family. No family vehicle or dynastic holder appears in the 5%-holder table or the officers-and-directors table, so unlike Simon Property Group there is no founding-family row to place inside or outside the officers group here.
• Register self-gate, computed on this final file exactly as the chart computes it (aggregate container reduced to its 8,693-share residual since its 15 named members are also present as their own rows, everything else at full value): 27 rows, total charted shares 144,045,355, against shares_outstanding of 297,702,812, i.e. 48.39% of the company. This is above the brief's typical 25-45% band but the brief itself notes REITs run to the top of the band because of heavy index ownership; the largest single contributor is the Vanguard family at 47,791,126 shares (16.05%), consistent in scale with BlackRock (11.28%) and State Street (5.92%).
• No em dashes used anywhere in this file per house style.